Well, I have 3, and would always keep a MT and a Buggy. Which ones? Not sure if it matters. I like my Tmaxx, but sometimes want other MT's like the Savage, or at the moment the LST. The Revo doesn't do it for me. They should have gotten rid of that POS 2.5 if they want to stay competetive. I also like my Lightning Pro 2, and also sometimes want other buggies like the MBX Prospect coming out soon, the K cars (II, III, or 777) and am a big fan of the Hyper 7 and Storm. As for the third I would trash my ST (since I never drive it anyway) for a onroad car in a heartbeat. Either the NTC3 Factory Team, or if you want to talk high dollar then I would say the Serpent 950R. That thing is sick!
Well that is what I like, and in short it would be (in this order):
1/8 Buggy
Monster Truck (any scale)
On Road (hopefully 1/8 scale)