Best years for KX 125?

2003-2005 are the best
2003 is a slight odd ball with the older style dog bone linkage and funky pipe mount

99-02 are not bad bikes, stable but do not turn the greatest
02's can be made to be very fast, but will have a very narrow powerband (I owned one)

if you can, get 03-05 really good chassis after suspension is sorted out and good motors

the older ones can be made good. they are heavier and stable on straights. Motors can be beefed up with some porting. I had a team green bike years ago and it was faster than my 05 yz125, but it was also tuned to be all top end.

I think the best year kx125 motor is when they added valves and 2 more strokes

LOL

03-05 are pretty good bikes. I have a 03 built into a 144 and the motor is super smooth and strong, best mod I did to the bike. If it had more over rev it would be a perfect motor for a small bore. Definitely stronger then my 07 CR 125. The shifting is not so good especially if the track is loose with a lot of tire spin, sometime it will not shift under WOT. The stock forks are harsh, shock not too bad, handles and turns ok to me. If I had the transmission worked to shift better under WOT, the suspension re-valved and gained a little more over-rev I would probably park the CR for a play bike, the motor is that good now. Its just frustrating to ride not shifting under WOT sometimes, it was less of a problem when it was a 125. The 03-05 cylinders are pretty much a copy of the YZ 125 cylinder