Man, that's a tough call. Each member of the band evolved as time went by, just in different spurts...
Eddie, I'd have to agree and say Fair Warning. Got a uniquely dark vibe compared to the rest of the six pack, and so many crazy, innovative, out-of-left field parts to it. Be it the tapping intro to Mean Street, the solo in Push Comes To Shove, how for the first minute of Dirty Movies you never quite know where he's gonna take the song, and then it swerves into that cool riff... Especially on the first few listens, the album kept me guessing, in the best sense of the term. Really inspired. I actually like his tone on Diver Down a bit better than on FW, but it's still his peak.
Alex, my vote goes to Van Halen II. The first song I associate with him is s still Light Up The Sky, one of my favourites in the catalogue. Outta Love Again is another one of my favourites for drums in a Van Halen song. You could probably argue that he changed it up a bit more on the records that followed, but those are the AVH tunes that stick with me.
David Lee Roth, I want to say Van Halen I. The way he lets loose with his screams on On Fire, Atomic Punk, I'm The One... Sure, there were a lot of squealers in the 1970s -- Ian Gillan and Robert Plant come to mind -- but none of them had the same energy or rasp that Dave did on the debut album. A close runner-up for DLR though, is 1984. Jump wouldn't have been as big a hit without his vocals, and he got to touch on a variety of songs with I'll Wait and Girl Gone Bad.
Michael Anthony, I'm leaning toward Diver Down. I can't imagine Where Have All The Good Times Gone? or Hang 'Em High without Mike's bass and backing vocals.