Aqualung and Warchild are both necessities; but that said, neither one is actually my favorite (still great records though).
My favorite of theirs remains Heavy Horses, but lots of fans differ when it comes to Tull (they went through lots of style changes).
Their first album,
This Was, is very much old school rhythm and blues (with a flute). The following two records (
Stand up, Benefit) continue in this style, but you gradually start to see them blend more of a hard rock/progressive style into the music.
Then comes
Aqualung, which many people recognize as their seminal album and really solidified their future style.
Thick As a Brick is a fantastic album, and really an incredible achievement. It's also when Barriemore Barlow began on drums, who I think is better than Clive Bunker (although some traditionalists might call this blasphemy). Some of my favorite musical moments of rock are on this disc. They followed that with
A Passion Play, which, like Thick As a Brick, is divided into two tracks but is essentially one long continuous song. It's not as accessible as Thick As a Brick, but it's still cool.
Warchild has more of a hard rock feel.
Now, we go to
Minstrel in the Gallery. On this album they started incorporating more folk elements into their sound, along with the hard rock. Minstrel is an amazing record, and might be my second-favorite.
Too Old To Rock n' Roll: Too Young To Die! is another concept album. It's good, but I think a slight drop-off compared to Minstrel.
After this we enter into their folk era.
Songs From the Wood might be my third-favorite album. They moved slightly away from the hard rock and opted for a more folk-influenced sound. Still, the songs are badass.
Heavy Horses follows in this vein, but I think it's their most consistent, solid record; really an example of genius songwriting. I never tire of this album.
Stormwatch is included in what is usually referred to as the "folk trilogy," but it actually is more of a return to hard rock (it just continues some of the lyrical themes from the previous two albums).
After Stormwatch, the lineup of the band changed severely, and this affected their sound in enormous ways (Barriemore Barlow quit, keyboardist John Evan was fired, and bassist John Glascock died soon after this record). Stormwatch is generally considered the end of the "classic" Tull sound. I wouldn't recommend getting any albums after Stormwatch, except
Broadsword and the Beast, which has some great songs on it.
And a word of warning: stay away from
Crest of a Knave, as it is a terrible record and it is blasphemous for any metal fan to own
(I've since sold my copy).
EDIT: I'm psyched about this thread, should be fun!