Everyone will have a different opinion. .21, .24, .28 it all depends on your driving style and which company you like. I know many drivers who love their V-spec's these engines are very smooth and reliable and easy to tune. I personally have had great luck with GRP but they are out of business so when these 2 engines are done I will switch to Nova. I like the new Toro from nova, but I like a lot of torque down low. I had my GRP's modded to specifically give more torque response, as GRP's are not strong on torque in heavy truggys, and the mod worked very well.
So consider the following:
- your driving style (too much engine can cause problems)
- your budget (some engines can get into the 500+ range)
- what you want from the engine (smoother, punchy, gas mileage)