Depends on so many things, what bass you use, what amp/cab you use, your playing style, your musical genre, size/cost limitations, etc.
If you want the 'best' 'distortion', the Brimstone XD-1 takes some beating, if you have the cash and the board space.
[url="http://www.brimstoneaudio.com/crossover-distortion.html"]http://www.brimstone...distortion.html[/url]
[url="http://www.prymaxevintage.com/brimstone-audio-xd-1-crossover-distortion/"]http://www.prymaxevi...ver-distortion/[/url]
[media]http://www.youtube.com/watch?v=6pHo6fPdPvM[/media]
The EBS Billy Sheehan looks cool too:
http://www.youtube.com/watch?v=8ch5ZIJgp2Q
But there may well be other pedals more suited to you depending on your needs!