I'd say the most odd and complex music is found on Six Degrees and ADTOE respectively. On Six Degrees they were doing all sorts of odd things like recording guitar solos backwards, using new keyboards, bass necks grafted on guitars...not to mention all the sonic experimentation. So in that way, it's pretty odd. However, with the exception of the Blind Faith unison and a couple bits in TGP, I don't find much of this album to be very difficult to play on guitar. So from a playing standpoint, it's not too complex.
ADTOE is the other way around though. As far as the recording and writing goes, it's really just DT doing their thing. The playing though is really kicking it up a notch, especially compared to the few albums before it. You've got Mangini doing his crazy polyrhythms, Jordan really going all out on his noodly solos, and Petrucci has some crazy licks on this album. The Outcry instrumental section is one of their most complex pieces, the Lost Not Forgotten tickle section is ridiculous, and the other songs (with the exception of BMUBMD and the ballads) all have little complex moments sprinkled in. I remember this being one of the most difficult albums for me to learn on guitar.
And of course you have to give props to Images & Words. DT may have surpassed themselves as far as complexities go since then, but at the time that was about as complex as Metal got.