If you want nippy then these are the fastest:
Cagiva Mito - Fastest stock 125 in a straight line, not so good for cornering or acceleration. Very high geared so massive top end for a 125.
Aprilia RS 125 - Very quick bike but 2 stroke and liable to blow up. Italian engineering + 2 stroke engine = disaster waiting to happen. Stick with the Japs if you want reliability.
I CANNOT STRESS THIS ENOUGH!!! -> IF IT'S CHINESE, WALK AWAY!!! IT WILL BREAK AND BREAK AGAIN BEFORE YOU GET A CHANCE TO REPAIR THE FIRST FAULT... BEFORE YOU KNOW IT THE BIKE IS SCRAP METAL... The chinese don't know their a** from their elow when it comes to mechanics. SERIOUSLY!!!
Honda NSR 125 - Very rare nowadays but also extremely quick... They don't have a massive top end but they'll rip your arms out of socket at the lights!
Remember, if it sounds chinese it probably and walk away...
As a general rule of thumb, if it aint Japanese it ain't worth s***