OK, don't laugh ... The lens I've found most useful for Panoramas is the 40mm pancake - the negligible distortion is a breeze to work with. In fact, I was guided to it by Sporgon here on CR - BTW, I really like his work.
I recently got the 17mm TS-E lens which has a stellar reputation. I'm still getting my head around the various tilt and shift movements so I haven't tried creating panoramas with it. The learning curve is extremely steep and I'm struggling with time constraints these days
Thanks J.R. Yes the 40 pancake is my default lens now, assuming no IS required. It has zero distortion and a short nodal point meaning that it (generally) stitches together very easily with little after corrections needed. I don't bother with a sliding panoramic plate anymore as the recent stitching programs are so good.
When in portrait the vertical field of view of a 40 mm is about the same as the vertical field of view of a 24 mm in landscape orientation, so people who like 24 mm are going to find panos with the 40 familiar territory.
However as Don says, it does depend on where you are and what you are trying to produce. I've attached a few pictures that range from 400 mm, 189 mm ( 135L+1.4II converter), 135 mm, 40 mm and 28 mm.
On the 189 mm shot I was on the side of a hill and couldn't move any closer without losing too much altitude. In the 135 mm shot I needed to get the trees in front of the minster out of the way as much as possible. By moving back the trees become smaller in relation to the building behind. Google images of Beverely Minster and you'll see how when you are closer the trees obscure more of the church. In the 40 mm shot it was the right framing for the perspective I wanted, and lastly on the 28 mm pano I couldn't move any further back for a number of reasons.
@Mack, not at all surprised on the 50L. I'm sure you are shooting in portrait so over lapping the long side of the sensor takes away the edge resolution. The sky takes away perceived resolution in the top part, but most importantly you are creating a larger format and so the enlargement of each frame is much less, and is much more forgiving on the lens. ( Just like MF / LF does not put nearly so much pressure on the resolving power of the lens.
I can honestly say that in my panoramics it is impossible to tell the difference in print between the 24-105L @5.6 - 8 and lenses such as the 35L or 135L at the same apertures.
I would never use a 17 mm lens for a panoramic.