The Who gets more emotional peaks out of me (probably from Townshend's lyrics more than anything else), but I find a lot more interesting material in Zep's catalog percentwise than in the Who's and I listen to Led Zep more, so Zep wins. Let's overanalyze this, two bands that are quite similar and still very different:
Members: Daltrey's much better (and less annoying) than Plant, at least on the Who's peak material; Entwistle outstrips Jones on bass, but Jones was more versatile as a bassist/keyboardist. Page rips Townshend on guitar, little argument there. Moon and Bonham are pretty much equal (drums-as-orchestra vs. hard rock bottom taken to its finest).
Music: The Who aimed for more emotion than Zep in their music, but Zep was more diverse (Houses of the Holy anyone?) and consistent in sticking to the hard rock that they did best. As good as the Who's best albums are, there's a reason every teenage boy interested in rock (me included) collects all of Led Zeppelin's catalog and obsesses over it for years.