This is so hard for me; it's almost a deadheat between Time and Comfortably Numb. They are both classic, well-known examples of David's guitar prowess, but on this one I have to go with Time, just because it's more upbeat and less moody than Comfy Numb.
If you had put Echoes on this list I think there's no way I could pick one.

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David Gilmour: a strat never sounded so good