2007 - the GI Joe team is so awesome - I kept them all out on my desk for two years. It was probably one of the best thought out and executed sets they ever had. I haven't even looked at the female vipers since I bought them - that set was all about the Joes. Doc, Steeler, Clutch, SkyDuster, they all rocked.
The 2008 BATS are my second favorite. Because they did a great job morphing the two BAT versions and maintaining my favorite parts of the original BATs. The 3-man SWAT team is pretty awesome as well. I was playing with Shockblast after lunch today coinicdently.
The rest of the sets are just blah. I have the 2006 set and its a dissapointment. Major blood and his troops are not bad, I just don't get as excited about them as I do other figures. The famed Crimson Set doesn't appeal to me because the Vipers with BAT legs look funny to me, always have. They have never had the same appeal to me as the original Vipers. The Drednok set was something fun and goofy - but you don't pay $300 for a goof.
A sleeper no one has mentioned is the green-shirt dusty parachute drop figure. That is the BEST repaint of that figure ever.
http://www.yojoe.com/action/02/dusty6.shtml I have him MIB but I wish I had one loose.