I've had a 2005 CRF250, that was a great bike. Handling was amazing, wasn't a powerhouse but pulled me around good for 3-4 seasons with no issues. I also had a 2007 KX250F, that bike didnt handle as good as the honda, but wasn't unridable either. Pretty much would have chewed up the CRF and spit it out the exhaust powerwise, these two bikes werent even close. It was like moving up to a 450.
Now I got 2002 RM250, Handles amazing and pulls my but around twice as fast as either 250cc four-stroke, and is 3 years older than my oldest four-stroke.
I have never ridden anything other than a early YZ250F (03/04 ?) and it was all mod'ed out so I cant give my opinion on those.
And KTM.... just takes a completely different spin. I only got to ride a 2006 250SX-F for a few laps and it didnt have me sold, I really honestly have no opinion on these other than they look badass.
Also, consider your size and statue in the buy aswell, if your a bigger guy like myself. 215 with no gear and 5' 10" I wouldnt go with any 250f for that matter... you'll always be wide open and looking for more gears. Where as its more enjoyable to ride and have the power to get over the bigger obstacles.
My KX250F was $2100 in parts for a full rebuild bottom to top (including cylinder). I got less than $500 in my RM250... so thats something to think about also.