without spending a lot of money on the drive train, the RB TM523 or the picco .26 max are the top two choices, if money is no object the sky is the limit, i have seen a few revo with STS D30M engines in them, freeze runs a revo with an modded novarossi 528 (probably the most powerful .28 engine there is), if money is no object the only limiting factor is how drivable you can make the truck.