Im afraid there's really no very easy answer for your question, it gets extremely complicated depending on what period of the war, what formations, planned replacement rates, etc. Ill take a swing at it at least piece by peace according to major armament systems:
Tanks/AFV Please keep in mind, that throughout the war there were dozens and dozens of different tank/AFVs, and during the period from 1939 to 1945 development quickly made a large number of them redundant or obsolete, so some vehicles had relatively short service times, which skews our understanding of reliability for alot of these vehicles. Realistically, the answer is probably the raft of obsolete and wholly inadequate Japanese tanks used in the pacific, but they were deployed in small quantities in an exceedingly adverse environment.
For the major powers using tanks in divisional quantities, id go German heavy tanks being the most unreliable overall, Tigers were first deployed to Tunisia in late 1942/early 1943, and along with its derivatives the Jagdtiger and Sturmtiger fought until 1945. The Konigstiger (or tiger II) was first deployed in Normandy in 44, and went through the end as well. All of these heavy tanks suffered from major production and mechanical failures, and often lost a significant number of tanks due to mechanical failure just moving from one part of the battlefield to another. The major causes for this were mounting a large, heavy gun, accompanied by heavy armor, but tied to inadequate suspensions and engines. There are two volumes of Tigers in Combat by Wolgang Schneider that have excellent unit histories on all the Schwere Panzerabteilung that operated Tiger variants, and the continuous maintenance and mechanical difficulties experienced by the crews both early on as the units first became operational, and later on as replacements and maintenance parts became scarce due to allied efforts to hamper German production.
For the best AFVs overall for reliability, its arguably the American M4 Sherman, it was a simple, robust design, mounted a simple, powerful set of engines (the Continental/Wright r975 at first, and then the Chrysler multi bank in the M4A4), had good road wheels, suspension, etc. More importantly, american formations were lavishly equipped with parts for preventative maintenance, take a look at the 1943 version of a Sherman service manual: https://archive.org/details/TM9-754, not only does it illustrate the relative simplicity of the design, but also details just how much material and organization was provided to support a Sherman in the field (read through the tools and parts section, detailing exactly who has a tool and where it is in the command structure). Sherman Medium Tank 1942-45 Steven Zaloga
Infantry small arms Quality was actually almost universally good, most armies utilized bolt action rifles with multiple-decade long design pedigrees, same goes for machine guns, nothing was overtly terrible, with one major country being the exception (bar a few individual non-standard issue weapons, im going to focus here on standard issue armaments). Japanese small arms were rather poorly designed, the standard infantry rifle was the type 38, which had two major faults, it fired a small 6.5mm round, and it was 1.2m or so long (adding its bayonet, its around 1.6-1.7m long.. longer than many of its users were tall!). Most of the Japanese light machine guns (all the Nambu series) also used the poor 6.5mm round (and most of them also included a bayonet mount... on a support weapon..). The Japanese type 92 HMG did go up to a 7.7mm round, but this was fed on a relatively short feeder strip, which needed to be oiled to prevent jams, which only made bigger problems, as dirt and grime inevitably got into the oil, causing in-opportune jamming. Japanese Infantryman 1937-45: Sword of the Empire Gordon Rattman
Aircraft This gets far more complicated im afraid, so i will keep it simple. Different weight classes, operational function, role, etc. all played a huge role on the reliability of an aircraft, a dive bomber had drastically different forces applied to it as compared to a level-bomber, or a transport aircraft. For simplified comparison, ill focus on fighters; as engine overhauls could go from anywhere from 50-200 hours of flight time, what really mattered here was the engine type. Radials were renowned for being easy to work on and having increased reliability, then in-lines, and finally jets. Aircraft like the A6M zero, the FW190, the P47, etc all had broadly equivalent reliability as fighters (all radials). The bottom of the pile, by far, was the ME262, which practically required overhaul after every flight (maybe 2 if they got lucky), this is almost solely a function of production limitations and the fact that it was the first operational jet engine fighter.
Edit: submitted early by accident