They're all designed very similarly as far as the motors 0 thus, the maintenance requirements are virtually the same.
Pretty much all the modern bikes are good. It basically comes down to your personal preference as far as handling characteristics, power delivery and color.
Ride a couple if you can, pick a color and go ride.
Oh yeah - I'd try to get the newest one you can afford.
I like KX's because I'm short at 5' 5" (I think Rob, up above, is right there with me, though!) and the KX's just fit me well. On YZ's I feel like I'm up quite a bit higher. I like RM's pretty well, too, but KX's just fit me. Haven't ridden that many CR's to say anything.
Maint-wise, I think YZ's are the best built, but I've had no trouble with either of my Kawi's and slightly more trouble with my Hondas than others bike.