I was wondering which pipe is best for motocross on a Fourtrax 250R. Paul Turner MX, Bills MX, FMF MX?

 

I hear goed things about FMF

 

I run an FMF Fatty. Pulls hard from the bottom and comes on strong thru the mid-range, with a little drop-off on top-end. I'm happy with the fit and finish of the pipe, as are a lot of my friends. Now, a lot of people don't care for it's color (it'll turn gold). It's also not that expensive (I got mine for $165 at Rec Unlimited in Orange).
LRD Adjustable is really a beautiful pipe, and it's tunable for you're riding style. It's got a really good chrome finish and a lot of people are saying good things about it. It costs more than the FMF, but it has the advantage of being tunable.