The most reliable, used 125cc bikes are from Husqvarna, Honda, TM and KTM, and Yamaha. Suzuki would be a runner up and Kawasaki would be the back of the pack, due to slightly lower build quality. They can be picked up cheap, in good condition, so will have good reliability.
On the 'Dirt Rider' forum, the Yamaha YZ 125 is raved about for reliability. The KTM 125 also is high up in the discussion. The older 125 bikes are mostly 2 stroke. There are good used 4 stroke bikes that are more recent and 4 stroke is generally more durable. The reliability of the bike is going to depend on how it has been and still is maintained and cared for.