Interesting that this thread is still alive. Short answer - 3mm "jazz" style picks.
As a jazz guitarist first and foremost, I mostly play with my fingers (as did Joe Pass and many other jazz greats). Probably because I first trained as a classical guitarist. Playing bass with my fingers feels so much more natural and, for me, more control over the dynamics. But I have heard many great bassists ply almost exclusively with a pick. I'm sure it has a lot to do with the style you're playing. All this said, I always keep picks in my pocket, because some songs just need that firmer, crisper attack (and a bit more assertiveness.) I almost always use 3mm picks: Dunlop Jumbo Flow, Dunlop Big Stubby, or Ernie Ball Prodigy. I'll occasionally use a Fender 351 Heavy (.96mm) which I like, but it feels a little alien in my hand.