I think there is no doubt about the best alternative to a real 303 is a x0xb0x, the sound, sequencer, and all things in a x0x are truely oriented to be a clone.
There is also good acid machines, maybe sounding like a real 303, but haven't the taste, philosophy... of the 303
Analogue Solutions TBX-303 / Trans Bass Xpress
Acidcode ML-303
Future Retro Revolution
Novation Bass Station (really cheap in the UK)
Doepfer MS-404