Are we talking about modern vehicles? Or old ones? I mean, that's a pretty big area, and you can't really compare the technology of the 40's and 50's with today's.
Here's MY votes:
Worst tank (pre-1950) - MAUS (too heavy, too slow, too restriced)
Best tank (pre-1950) - T34/85 (arguably best tank of WWII, easy and cheap to manufacture, a dummy could operate it effectively, reasonably well armored, automotively reliable, very combat effective)
Worst Tank (post-1950) - M60A2 (too far ahead of it's time, dangerous for the crew, too large, used the faulty 155mm Sheleileigh (sp?) missile system)
Best Tank (post-1950) - Tie between T72 and M1A1.
Everyone simmer down a sec. The M1A1 for obvious reasons, but the T72? Sure, here's why: very inexpensive to manufacture, very well armored (for when it was fielded), excellent penetration, small, low profile, fast, dummy-proof, 125mm. The only drawback to the T72 is it's short engagement range and it's autoloader.
\"Each man will charge forward to the very end, irrespective of the cost in casualties. There will be no halt and no retreat. There will only be the assault and the advance. - Gen Israel Tal Israel,1967\"