If you ask 10 people the same question you're probably going to get 15 different answers, as various threads on this board will tell you. I've learned 2 things for sure to answer this question. First: no matter how fast your engine is, if your setup isn't right (car, clutch and exhaust) and you can't drive, you're going to be slow. Second, no matter what engine you buy, there is ALWAYS going to be someone fatser than you. It's just how it is.
That said, it's very hard to go wrong with any of the current engines out there, be it Nova based (353 or +12 something), Ninja, OS TZ, Picco (Jl Red Dot), Sirio, or even the Orion CRF. Each has it's pros and cons, each has their supporters and each, tuned correctly, will last for a while and be stupid fast.