I have participated in every game since 2001, and here are my thoughts on the games that I have observed.
Ultimate Ascent (2013) and Rebound Rumble (2012) were heads and shoulders above the games that came previously (or since). Excellent mix of scoring challenges, a good offense-defense balance, linear scoring, a nice mix of autonomous challenges, fun to watch, AMAZING endgames, some mobility challenges to consider (going under/over obstacles), worthwhile vision challenges, relatively easy to explain. Oh, and relatively open parts utilization rules compared to prior seasons. Some nits to pick (like the Coopertition Bridge aspect in 2012, and the fact that climbing should have been worth more points in 2013), but all around excellent games.
Next I put Aim High (2006). It was a simple high volume shooting game, and the first time the vision challenge was worthwhile.
FIRST Frenzy (2004) was a nightmare to explain, but it was a fantastically intricate challenge with many ways to score and led to highly varied designs.
Triple Play (2005) had an elegantly simple game mechanic (tic tac toe) and was always exciting to watch (though they should have done something different with the endgame).
Breakaway (2010) was a great idea, but would have been executed better with more lenient robot rules regarding ball possession.
Rack N Roll (2007) too often degenerated into a defense-fest, but it was a nice scoring challenge and a really interesting endgame. Would have been better with linear scoring and some sort of mechanism for limiting defense.
Overdrive (2008) was exciting to watch, but a little too simplistic for my liking. Would have been better with an additional game piece or a more heavily emphasized endgame objective.
Aerial Assist (2014) was awesome during Championship eliminations, and painful to watch just about any other time. The rules led to too much referee subjectivity, the autonomous challenge was such that you were better off not to try than to try and fail, and 4 out of 6 robots had no offensive objective at any given time, leading to scrums. I maintain that the game would have been MUCH more interesting if the truss had been only 18" off the ground
Logomotion (2011) remedied the defense balance of 2007, but maybe a little bit too much. The endgame challenge was the worst that FIRST has ever given us, and the scoring system was terrible. The IRI rule changes made the game a little more palatable.
Stack Attack (2003) had the original autonomous mode, and some interesting strategic aspects due to the nontraditional scoring mechanism, but was once again totally broken by having too much defense (and said nontraditional scoring mechanism).
Zone Zeal (2002) was a stump pulling contest.
Lunacy (2009) was like watching children play rugby.
Diabolical Dynamics (2001) is alone at the bottom because without robots interacting, who cares?
(I will not rate Recycle Rush until after the season, but you can probably infer my opinion from my ranking of similar games...)