Upgrade with most bang for the buck?
 

In my opinion it has to be the NiMH batteries... even without the bearings and springs... it's night and day different with these cells. I tried them on my brother in laws stock red Supra and it was ridiculously different from alkalines.

Mine would have to be the stage 2 motors. They are definiately worth the money. And it wasnt that hard to install.

i like all of the upgrades.