Look at the ones the other guy listed for sure, but DO NOT leave out Husqvarna and TM, TMs are the most race ready bikes out of the box, more trick parts than a KTM could dream of!!! TMs are all hand built in Italy (think the Lamborghini of motocross and endurocross).
If moneys not an issue, go with the TM, Husky, or KTM, their 250 4 strokes are unmatched out of the box.
If you have a price limit, go with your favorite color, a yz250f, rmz250, kx250, and crf250r. Japanese bikes are essentially all the same other than plastics and ergonomics.