[Racer26]

There's been many drivetrains over the years I've really liked (some decidedly more successful in practice than others)

2003: 45 built a "Ball Drive" that worked a bit like an old ball mouse, but instead of the ball being the input, it was the output. (IIRC this wasn't their competition bot, but rather a proof of concept)
2003: 1075 built a "reverse snowmobile" (two undriven wheels at the rear of a wide orientation bot, and the front was a steerable unit not unlike the belt of a snowmobile)
2006-2008: 1075 built several incarnations of a belted longbody tank drive, with the CIMs inside of the belt.
2010: 1625's 6-wheel swerve
2010: 1075's Invertable swerve
2010: 1114's PTO-lifter
2011: 148/217's Nonadrive and its variants

Another drive concept I'm not sure of its origins:
Kiwi Drive (3 omni wheels, 120deg opposed)