Thats funny how different people like different albums as I'd rate Lateralus as the best, and probably one of my all time favorite albums, and 10,000 days as second then undertow and Enema as my least favorites. Having seen them live at OzzFest a few years back they rate as the single best live set I have ever seen as well.
Each to their own i guess.
I can see why Lateralus is rated highly. The way all the tracks go together so seamlessly. If your a fan of Lateralus then I would recommend listing to Lateralus - The Holy Gift. Its basically Lateralus re-ordered into the Fibonacci sequence. Without going too deep there's a lot of strange time signatures hidden within the tracks that fall very oddly into the Fibonacci sequence when re-ordered.
I agree Lateralus is an amazing album, absolutely. But pretty much their whole sound changed between Enema and the release of it. Its strange to hear a Tool fan rate Lateralus above the previous 2 though. Enema is highly regarded as their best album. So many good tracks on the first 2 albums!
To name but a few:
Undertow -Sober, Prison Sex, Bottom
Enema - Push it, Stinkfist, Eulogy, Jimmy, H
All classics, but yeh, I really dislike 10,000 days, lol.
And to echo what others have said, please do come to the UK, Tool. I would die a happy man if I get to see them play at least once in my life.