I'd say
Unicorn drive is up there. There's so many modes available: tank (uncommon if snake works), full crab (all wheels pointed same direction), snake (front & back point opposite directions), center spin, arbitrary center spin, each potentially with x and y-orientation. The real trick is switching "modes" on the fly. Some have 2 speeds, either mechanical or software-based (less optimal but alright for some games/strategies).
We've been iterating 4-wheel swerve for 2 years (without tank mode). Heck of a ride. Go programmers!
The other possibility that comes to mind is Nonadrive. It's incredible, but I have no idea how complicated the programming is. (148, are you here?) Of course, there's always auto-balancing trackballs. Or triple-jointed six legged bots.