2004, by far.
I would have put to use the knowledge we've gained in the area of tank treads since then to make sure they wouldn't break during the most inopportune of times. I would have also found the extra weight to install the big ball gripper mechanism we had designed but couldn't use.
I am also somewhat proud to claim we tried to bring back the 2004 game ourselves this year - according to some observers, after we hurdled once at Midwest our robot caught air and swung on the overpass, effectively hanging on the bar.
The lift design we used this season is also a variant of our 2004 design.