I would recommend either an '00 or an '02 YZ 125. I would stay away from the '01 because it was the first year they went to the 38mm Mikuni carb and I wasn't too happy with my '01 because of that. It was a bog monster down low and a jetting nightmare.
From my experiences, the '00-'02 CR 125's were total DOGS, the '00-'02 KX 125's were okay but lose that "solid bike" feel quickly and develop rattles and vibrations, and the '00-'02 RM 125's had some clutch and tranny issues.
Seek out a good used '02 YZ 125 or '00, skip the '01 for above reason's. '00-02 KX's are good bikes but avoid abused ones. I'd recommend the '01-02 RM's perfer the '02's cause the fact that they fixed the power valve operation acutating arm. The 01's had a bad design that could break off and take out the tranny or other bottom end parts but you can easyly swap to the '02 parts for about $35. The '02's also got the newer tapered swing arms and excel rims stock. Stay away from the CR's!
Ok, just to play devils advocate, the '01 YZ is a great bike! Really, I have no jetting issues at all in stock form. Its very forgiving on jetting based on temperature, runs great from 40 degrees out to 85. (Can't complain there.) The 38mm carb is notorious for making very little bottom end, however, that comes with a very distinct advantage: incredible topend power. If you riding skills include being good at staying on the pipe, the powerband issues won't hurt you at all. And it is a fact, in repeat tests, my '01 YZ125 will pull ahead and stay ahead of my buddies '03 KTM SX125, both in stock trim. So the YZ is making hella power! Great suspension, 6 speeds in the gear box (That would steer me away from an '02.) The KX in 2000 is a pretty decent bike, good motor. No experience with the RMs, and the Honda CR125s of the last three years, well, I've heard LOTS about issues of lacking power.
I can only tell you about my own bike, but I really love my 02 KX. It wasn't the quickest stock, but it's really easy to make quicker, just throwing on an fmf fatty made a big difference. It's not picky at all as far as jetting goes. It's been totally reliable.