Aenima is probably their most straightforward (and I mean that in a good way)
Not more straightforward than Opiate or Undertow.
Opiate perhaps but people tend to forget that Undertow was actually still fairly proggy, the title track being a case in point.
None of this proves it to be less straightforward than Ænima. It's still more straightforward by the manner in which it doesn't have filler tracks mixed into the tracks but only as a silly and lengthy wait until the (not-so-)hidden track. Also the prog argument is completely turned against your comment when you look at Eulogy and Third Eye alone. Beyond that most Undertow songs are based primarily on riffs and don't have a bunch of atmospheric dynamics interspersed into the songs but only passingly as a brief intro like in the case of Prison Sex.
I'm not saying it's as proggy or proggier than Aenima at all, I'm just saying people seem to think it is ENTIRELY riff based and in 4/4 et cetera. It's not, it's more complex than people think.
Side note: I can't decide whether 10,000 Days or Lateralus is better musically (the albums, not the songs). Because 10,000 Days has Vicarious and Jambi which are two of their most complex works, and Right in Two has some nice polyrhythmic stuff at the end. But then Lateralus has Schism, probably their most complex arrangement, the title track and Ticks and Leeches. I'd probably give it to Lateralus, but bear in mind how CRAZILY complicated Jambi is, I never realised until I tried to learn it on guitar and realised it was polyrhythms galore.