I race a YZ125 and I love it, it's a very good bike.
It depends though, are you going to be on trails or doing motocross?
The Yamaha TTR an awesome trail bike, but Honda's are pretty good also.
But as far as motocross goes- the YZ125 has been the best for years now, statistically speaking.
Mainly though, go with what fits you best, whatever you feel most comfortable on.