The yamaha 450 is the fastest w/the stiffest suspension. The honda 450 is the all around best from what I have seen(You can pick em up for around 4g). My wife has a new LTR450 and it came stock almost ready to race w/+2 a-arms, but It was insanely expensive$$$

(9g). I have never liked Suzuki(slow hoochie). It is a real pain in the arse to repare==You have to take the whole fender off to change the radiator fluid(fun).
All the new 450's have a shorter piston skirt for higher speed(They don't last like the old 4-strokes, but are way faster).
On my wife's 450 the header tube glows RED after 10min. of idling(She burns her boot allot) Imo...I think 2-stroke atv's are cheaper and easier to fix. Maybe a TRX250R(2-stroke). My wife had the TRX250EX and it was like a turtle=It wouldn't even make it up small hill climbs w/ softer dirt(It has the long lasting engine though).
Imo...I would stick with the best HONDA.