My MX bike of choice is a '08 or newer YZ125 lightest in its class, broadest most useable power as well. Ktm's are cool, but they don't handle the rough stuff as well as a yami. They can be made to work, but it takes a knowledgable suspension shop. I've never liked the 250f's, there faster, but a lot more boring to ride. A good rider can make a 125 work, but you will get stomped if your competition is comparable in skill.
What workes for me, probably wont work for you, so my advice....ride 'em all and make up your own mind.