That's a good topic!
For me, I really can't decide on only one because there have been so many timelessly awesome ones. As Brickdoctor already threw in, the Forestmen are fantastic. Also, I really love the classic ninjas! The redcoat admiral is pretty much at the top of my "best minifigs" list. Additionally, the classic town pilot of this set is fabulous.
So much about the older seasons.
Speaking of the rather new ones, there are a few genius ones aswell: in Star Wars the Stormtrooper, Darth Vader of course (great fig!), the old Episode II clones (even though I don't own any, yet will certainly do sometime, they seem to be really nice) and the expensive Jango Fett. The new Boba Fett (along with the new Death Watch mandos, as it seems) is really, really, really great too. Oh, and do not forget the yellow-striped Phase II clone (Starcorps)! Maybe not just as classy as Darth Vader and comrades, but definitely a glorious fig.
The best minifigs outside from Star Wars would have to be Indiana Jones in full gear and... ยด*drumroll* this guy... no, just kidding, I mean the Spartan of course. I'd almost say the Spartan is the best fig ever. I don't, though, because there are all those others that can only be described with Bang! too.
Cheers! Let's let the discussion rage.