The only software that comes reasonably close to doing the 303 sound is the audioRealism Bassline. Althought hat still doesn`t quite do it properly.
However, fo that control/london acid city, wide phat ass sound, there`s a number of things to do.

Square wave gives the nice round full sound.
Bung on some chorus for extra phatness.
Antares tube plug in, as it`s all about overdrive and not distortion.
Accent command. It really is the feature. On audio realism I think any note velocity over 100 triggers the note accent.
Note slides also work wonders.