By far my most used lens on the 5D. I just sold my 35L today because I haven't used it in months.
But my opinion is to buy on the focal length, and use what works for you. Obviously what works for me is not what works for everyone who answered above me. All just personal preference.
The Sigma 30 will vignette terribly on a 5DII, so even though the optics are excellent, it doesn't work on full frame. It does work on 1.3x bodies, though I found the MFD too limiting on my 1DII, so I'm selling mine and I picked up a 35L, which is simply stellar.
I don't own a 50L, but the 35L is a real gem on the 5DmkII. Especially with the peripheral lighting correction, you pretty much can cut all traces of vignetting at f/1.4 effortlessly ( that is if you want to. As the vignetting does add a nice touch and pleasantness to many shots.)
The MFD of the 35L is also very handy for close up shots.
having shot with both, and owning the 35L - if it was either/or, i'd go with the 35L (shock, right?). wide enough for street shooting, but compressed enough for indoor portraits.
If there were a 50mm FL lens as good as the 35L I'd be all over it; but there's not, so I've learned to love the 35L until Canon copies it in 50mm length.
I use the 50L and 135L and find they work very well together. On the other hand, 50mm and 85mm are a little too close for my tastes.
If you already have the 85 and like it, I would suggest the 35mm over the 50mm. That recommendation is based strictly on focal length, not on any sort of optical performance characteristics.
