I've only been on FIRST for 3 years, so my favorites go as such:
2014 Aerial Assist. Gosh was it fun. When driving you were having fun and able to work with other teams and last minute moves could mean a win or loss for your alliance (I was the operator and in one of my alliance's semi-finals matches I released a truss shot right before the timer went off and it gave us the extra points to win the match... you could say I was excited
).
2015 Recycle Rush. I think this year is going to be a good one. I think that it will be more strategy and selectively choosing alliances. Especially in the playoffs when top seeded alliances will probably go for teams that can do what their robot can't instead of three amazing robots that could do the same exact thing and one being downgraded. I like that your robot almost has to have a specialty to do well. Should be good.
2013 Ultimate Ascent. I don't know, I didn't love this one. It was more of an in the air game than down on the robots' level (shooting frisbees high with no passing or such interaction like Aerial Assist). It is definitely my least favorite so far.