As /u/CatnipFarmer aptly points out, "most feared" is a nebulous and somewhat subjective term that elides easy quantification. For example, the U.S. Army Intelligence and Threat Analysis Center produced a poster The Soviet Big 7 in 1981 as a training aid. While the title text is alarmist, calling to mind police wanted posters, the intent of the poster was to familiarize personnel with these seven weapon systems and treat them with respect. Fear was certainly a component in the poster's pedagogy- respect the enemy or be killed by them- but not a dominant one. Other Western Cold War publications like The Threat: Inside The Soviet Military Machine by Andrew Cockburn or the DIA's publication Soviet Military Power-pdf warning for 1985 edition tacked towards a somewhat hyperbolic estimation of Soviet military capabilities. But such attitudes often had more to do with marketing, to either sell books or a military build-up, than genuine terror. Given that the Cold War never went hot, "fear" was often rhetorical and based on estimates on what might happen if the balloon went up, not actual experience. There was one weapon system that managed to create a good deal of apprehension on both sides of the Iron Curtain during the latter stages of the Cold War: the cruise missile.

The first generation of cruise missiles were large and ungainly descendants of the German V-1 and various drones, but technological progress by the mid-1960s had made these weapons increasingly dangerous. For the US and NATO, the primary threat from the cruise missile was a naval one. The "missile mania" of the Khrushchev era had led to the Soviet state pouring in a good deal of resources into developing and fielding a number of surface to surface and air to surface cruise missiles such as the P-15 Termit/SS-N-2 Styx. These weapons could be mounted on relatively small vessels and were by the standards of military equipment, quite cheap. The high speed and relatively small profile of the Soviet cruise missiles made them difficult to intercept with 1950s and 60s air defenses. The loss of the Israeli destroyer Eilat to Egyptian Komar-class missile boats showed the vulnerabilities of existing surface ships to this type of attack. Existing Western AA mounts were too slow to engage these cruise missiles and the loss of the Eilat and the proliferation of Soviet-designed missile boats prompted the development of Close-In Weapons Systems (CIWS) like the USN Phalanx and longer-ranged air defense systems like the F-14/AIM-54 combination that could engage Soviet bombers before they could launch their missiles.

But as Western air defenses improved, so too did cruise missiles which became smaller and more agile. The Falklands conflict underscored the cruise missile threat as French-designed Exocet missiles had the real potential to derail the British campaign to retake the island. The Argentinians only had a limited supply of air-launchable Exocets - five- but this limited number of missiles posed a severe risk to the RN task force. Had the Argentinians had their full allotment of twenty purchased Exocets in 1981 then it is possible that the RN would not have been able to retake the Falklands without significant casualties.

The Exocet was not an outlier in international development of the antiship cruise missile as they all became more deadly and ubiquitous within the world's navies. One of the fears engendered by the emerging cruise missile threat in the 1970s and 80s was that they could be a cost-effective form of asymmetric warfare in which fairly expendable launch vehicles like missile boats or aircraft could manage to disable or sink a strategic target like an aircraft carrier. There were a good deal of public and internal analyses of the Falklands conflict and its ramifications for a Cold War going hot. One of the problems that the antiship cruise missile brought to the table was that it drastically cut the defensive response times for air defense. The near loss of the frigate USS Stark to an Iraqi Exocet during the Iran-Iraq War was an example of this and the fear of being "Starked" likely contributed to the USS Vincennes's shootdown of Iran Air Flight 655 in 1988. The Soviets' fielding of a new generation of cruise missiles like the P-500 Bazalt/SS-N-12 Sandbox and P-700 Granit/SS-N-19 Shipwreck as well as new launch vehicles like the Kirov-class battlecruiser or Oscar-class SSGNs not only represented a qualitative leap over prior generations of Soviet cruise missiles, but also would be fielded in greater numbers. The Oscar-class's predecessor, the Charlie II, carried only 8 cruise missiles while the Oscar had a battery of twenty-four. There was a fear, and one that carried over into wargames like Harpoon or technothrillers like Red Storm Rising, that the Soviets could simply spam an American naval task force with conventional cruise missiles.

One of the advantages of the cruise missile as it matured was that it could contain either a conventional or nuclear warhead. This made the cruise missile a thorny issue for the strategic arms limitation talks of the 1970s as both superpowers claimed their cruise missile development was for conventional means. The idea of a nuclear-armed cruise missile was one of the emerging fears among Soviet leaders in the late Cold War. The smaller size and increasing accuracy of cruise missiles that vexed Western naval planners cut both ways for their opposites in the USSR. Soviet air defenses were formidable, but still had to contend with defending air space the size of a continent. One of the main fears was that the 1970s-generation of cruise missiles would be able to evade Soviet defenses as they were able to follow nap-of-the-earth flight paths. This was not merely an academic concern. In 1987, the civilian aviator Matthias Rust was able to pilot his Cessna F172P into Red Square. The Soviet PVO was only able to periodically track Rust, but command friction and other problems meant he was able to fly to Moscow without an interception.

The Rust affair underscored the difficulties of air defense for an area as large as the USSR and if a Cessna could evade air defenses, then supersonic cruise missiles would be a much harder target. The Americans' fielding of the BGM-109 Tomahawk cruise missile posed a dilemma for Soviet air defenses. Tomahawk cruise missiles' relatively difficult to intercept, portable, accurate, and could field a small nuclear payload. The missile's small size meant that smaller platforms like attack submarines could become strategic nuclear platforms. This made it much harder for the Soviets to perform counter-force attacks as the potential nuclear platforms the West could field increased. While the Tomahawk's W-84 nuclear warhead had a much smaller yield than its ballistic missile counterparts, the relatively greater accuracy of the BGM-109 could use its nuclear payload more effectively. One of the major fears of the Soviets in the latter Cold War was that cruise missiles could be used for a decapitation strike that knocked out key command and control centers. The smaller nuclear yield of cruise missiles also engendered the fear that this was a way for the West to create a politically acceptable form of nuclear warfare in which smaller but more accurate nuclear weapons could cut off the Soviet head. This was somewhat disconcerting for a generation of Soviet leaders whose major formative experience was the German invasion of 1941.

The dual responses to the cruise missile do show something of a Cold War convergence between the two sides. Both West and the Soviet reacted to the maturation of the cruise missile as low-cost weapon that would undercut the basic principles of their defense strategy. Whether it was Western naval power or Soviet command and control networks that came under threat, Cold War military and political leaders did fear that the cruise missile gave the other side a dangerous advantage. By the same token, both sides' development of ever more effective cruise missiles validated these fears creating a feedback loop where planning for future conflict had to take into account worst-case scenario projections of cruise missiles' capabilities.

Sources

Freedman, Lawrence. The Evolution of Nuclear Strategy. Basingstoke: Palgrave Macmillan, 2003.

Spector, Ronald H. At War at Sea: Naval Warfare in the Twentieth Century. London: Penguin, 2002.

Zaloga, Steve. The Kremlin's Nuclear Sword: The Rise and Fall of Russia's Strategic Nuclear Forces, 1945-2000. Washington, D.C.: Smithsonian Books, 2014.

“Most feared” is kind of a nebulous definition, but I would imagine that chemical weapons scared the hell out of a lot of people.

When the US built its main facility to dispose of chemical weapons they did it at Johnston Atoll. Take a look at a map, it’s hard to get much further away from human civilization than that.

The US removal of chemical weapons from Germany at the end of the Cold War was called Operation Steel Box. I’m on mobile so I don’t feel like writing out all the details, but the security precautions included the use of two nuclear powered warships as escorts so that the ships carrying chemical weapons could sail from Germany to Johnston without ever having to stop for refueling. They also avoided the Panama Canal and went around Cape Horn instead to avoid potential security risks. That should tell you something about how feared chemical weapons were.

There was a great deal of fear in the late cold war at the increasing accuracy of medium and intermediate-range ballistic missiles. A good example would be the RSD-10 (NATO code SS-20 Sabre), which was definitely a destabilizing weapon, as it was not only impressively accurate for a Soviet missile, it was also road-mobile and possessed a range that allowed it to sit in secure sites in the Urals free from risk of counterstrike, and gave Soviet forces a very credible first-strike capability against NATO airfields, nuclear weapon storage and dispersal areas, command and communications nodes and transport hubs with little warning. Previous Soviet medium/intermediate and tactical missiles had been generally regarded as less concerning due to factors such as poor accuracy (high circular error probability) poor deployment times and limited mobility and range.

The NATO response was to deploy weapons which caused as much fear and trepidation in the Soviets, namely the BGM-109G ground-launched cruise missile and Pershing II medium range ballistic missile. The road-mobile BGM-109A was deployed in hardened bunkers in 6 locations in Europe to be rolled out and camouflaged in pre-selected locations in times of increased tension. Although a cruise missile would take far longer to hit Soviet targets, it was feared as a highly accurate, long ranged and very difficult to intercept weapon.

Pershing II was greatly feared also due to its astonishing accuracy, but also due to a new ability to carry an earth-penetrating W86 warhead which combined to make it a highly destabilizing weapon, as Soviet leadership worried about its application as a decapitation or first strike weapon against Eastern European and Russian command bunkers, missile silos and even continuity of government facilities in and around Moscow meant to protect the Politburo from attack.

Thus both sides made vociferous complaints and exaggerations about the others intermediate nuclear capabilities and intentions. Combined with strong disapproval by various anti-nuclear groups in Europe about the deployment of the two new weapons platforms, these concerns would eventually inspire the signing of the Intermediate-Range Nuclear Forces Treaty in 1987 to cut down the numbers of these weapons.