OGT here (summer of '92 when they opened for Rollins Band).
I agree with the majority of comments here that seem to rate Lateralus as their best and overall 10,000 Days as their worst. I really hate 10,000 Days, it's a wanky concept album that really falls apart and takes itself even more seriously than other Tool albums.
Lateralus though is a great listen, one of the very few long concept records that really fuses well together and sounds good from beginning to end.
I can understand how newer listeners might not be able to catch Opiate and Undertow, both are products of a much angrier, younger band still feeling out their sound. Though Undertow is still a very complete expression of the band, I can see how some might compare it to the later more sprawling sound of AEnima and Lateralus and think it too bare bones or raw.