Having only been in FRC since 2010, my personal experience puts Breakaway at the top of my list, simply because it is what kicked off my career and was incredibly simple to grasp, yet also offered many strategic options (469, 51, 2337, 125 [cookie to anyone who can point out the similarity between these
])
After that, I would say that Rebound Rumble is a close second because of the action that the matches provided, especially the balancing. I can still hear the Archimedes side of the field exploding once Wave's alliance fell off the bridge and shot the Eh Team on to Einstein.
Of years that I didn't compete in, Overdrive and Aim High had always stood out to me, though I sorta got my Aim High fix through this year (Though it wasn't quite the same)