cr450f is supposed to be the best, but I haven't ridden it.
KTM 520 is a great bike but the power is very, very smooth, which some like some don't.
yz426 power growls and snarls etc. more of a rip up the track style.
Pick your posion, for me I'd try the cr450, it's supposed to have 2 great qualities the others don't, alot less compresion braking, (better for jumps and turning until you get used to it) and its supposed to be easy to start, though so is the KTM.
good luck :)