It's got to be Comfortably Numb for me, which is not only IMO the finest song on The Wall but also a fantastic example of the music Waters and Gilmour could produce when they were working together. As great as the song is as a whole, the second guitar solo is my personal highlight and one of my absolute favourites. It's also the first Pink Floyd song I ever heard in full, which gives me a little extra fondness for it.
I was underwhelmed with Kashmir on first listen, it's grown on me a fair bit since although it's definitely not one of my favourites by Led Zeppelin - I think it was an interesting idea, it's got a good heavy sound and starts very well but I find it becomes a bit tiresome towards the end and after listening to it I usually want to put on one of their more straightforward rock songs like Black Dog or Good Times Bad Times.
Both are great. However, here in the UK, talent shows such as the X Factor tend to overplay Kashmir, so, due to this overexposure, I'd have to pick 'Comfortably Numb'. Its better when taken within the greater context of 'The Wall', or so I believe.
I agree about the overexposure of Kashmir, the first time I heard it I found myself thinking 'heard this before' (can't quite remember where from though, I don't watch the X Factor or any shows like that), by contrast when I first heard, say, Dazed and Confused or When The Levee Breaks they sounded new to me.
Comfortably Numb does work even better as part of the album I find, after several tracks dominated by Roger Waters it's great to hear David Gilmour's vocals on the chorus and then hear the first guitar solo - and that's before getting to the seminal final guitar solo, which also signifies Pink's descent into total madness which leads on to the events of side four.