I voted for TTA. COP is a close second though. Too bad it most likely won't be operating during my visit. I've never been on Buzz Lightyear. It looks like a lot of fun though! I'm going to have to check it out on my upcoming trip!
For pure fun and repeatability it would be Buzz Lightyear. We also do the people mover many times but that is more for relaxing and slowing down. The sentimental favorite is COP but we would not do it more than once or twice if it would ever be open when we are there. Been there 4 times in the last few years and it was only open the first time in Feb 2001. Even late June 2003 it was closed.
I'm looking forward this year to riding it again for the first time since 1992!!!! My daughter has been too young the past two times we were at WDW and this year we will be going with my parents, brother and his family, and sis and her family. If my daughter is too scared to ride, she can sit out with my mom.
Also looking forward to my first e nights and hoping I'll be able to ride space mountain over and over again!!!! Gosh, I feel like a kid!!!