Although its use is often overstated in historical fiction, and everyone that played Medieval 2 and their brother already knows about it, the Zweihander did see some combat during the late medieval and early renaissance period. These great swords were built up to sizes as large as two meters long, and were mainly manufactured by German, Swiss, and northern Italian smiths. They weren't particularly common, and were mainly used by mercenary units performing the duty of anti-pike infantry, as the swords were supposedly effective at breaking the heads off of polearms.
Source: http://www.thearma.org/essays/2HGS.html
If we're talking about sheer length, however, the Macedonian Sarissa pike is a strong contender.
Pikes, polearms and 2 handed swords standing at about 5 to 5 1/2 feet long.
As armor got better and better the quality of the blade mattered less and less as simple physics became more and more important. A pike might not be versatile, but all the versatility in the world doesn't matter when you just need to break a cavalry charge, and you're standing in tight shoulder-to-shoulder ranks.
Similarly, unless you wanted to throw an entirely impractical amount of armor (even field plate generally wanted to deflect blows rather than absorb them. Your inherds became jelly if a strong enough blow hit it, penetration or not) on someone, the thickness of their armor would never stop a punching instrument, like a pointed hammer or a pickaxe style weapon.
