I prefer Zeppelin slightly to The Who, but it's close. I rank them this way.
1) Houses of the Holy
2) Quadrophenia
3) Physical Graffiti
4) Led Zeppelin
5) Led Zeppelin II
6) Who's Next
7) The Who By Numbers
Led Zeppelin IV
9) Tommy
10) A Quick One
11) The Who Sell Out
12) Presence
13) The Who Sing My Generation
14) Who Are You
15) It's Hard
16) Led Zeppelin III
17) Coda
18) In Through the Out Door
19) Face Dances
20) Endless Wire
Not to go back too far in the thread, but I wish I had a chance to call in for Who chat last week. I like Endless Wire more than Face Dances (the WORST), It's Hard (half OK half horrible) and Who Are You. To me it's on the same level as Who By Numbers- not top tier but I still find myself listening to it all the way through fairly often. It also reminds me of the tone of By Numbers except instead of being an angry drunk Townshend is an angry old guy.
The discussion on the show was very interesting, and after thinking about it it was surprising to me that one of my top 5 favorite bands had so few albums that I actually classify as 'great'. A relatively low batting average as Tom would say.
Not restricted to studio albums, I'd say Sell Out, Tommy, Who's Next, Live at Leeds, Meaty Beaty, and Quadrophenia are essentials. I'm even on the fence about Tommy. The 'song' songs are great, but whenever I hear it I just think about how great it could have been. The engineering leaves something to be desired (the story is that they left space for orchestral arrangements but ran out of money), and I think I would like it more if it sounded raw like Sell Out or the live performances or polished and synthy like Who's Next.
The Who will always have a spot in my heart for being my first real music obsession. It also helps that I got into them at just the right time in my life. Quadrophenia is the perfect album for when you're 14 and your parents are getting divorced.