With all the electric bass soft synth samples I've used, Cakewalk's SI Bass is the one I keep returning to. It's a bit quirky now and then but the sound is more convincing than anything else I've heard. I also like the "slide" feature which makes bends and slides a breeze. I think every soft synth should have that feature built in.
:)

I like it, too, especially the slide feature. The distortion's not bad, either. And it's the most resource-efficient sample-based bass I've ever used.
 
But I haven't used it in a long time because it's so squirelly in the high notes. The tonal shift between low strings and high strings is too drastic. But if you have a part that stays in the lower register, it's a fine instrument indeed.
 
My go-to bass is Orange Tree Samples' Rickenbacker. Now, if Greg could just adapt his slide expertise (as displayed in his Lap Steel and Acoustic Slide libraries) to the Rick, that would be total awesomeness. You're right - all basses should have that feature.