Personally I'd say Dream theater , because of all the odd time signatures the level of technique in there playing , polyrhythms, polymeters etc...

who do you think are the most complicated?

I know some of you are gonna say Meshuggah but I'd disagree
Dream Theater, there's no contest. They're my favourite by the way so I may be leaning a little towards them but when it comes down to it, who else can keep up with them in terms of technique, composition, speed, and absolute mastery of their instruments? There isn't another band I've found with such a line up of amazing musicians.
how about nightwish, for the sheer number of instruments they use in all their songs? imagine writing it all out in score.
STEAM: beachhhhhhhh

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What about Planet X? Sherinian is ex-DT and I think without DT his career wouldn't have gone the direction it did. Virgil Donati is practically a human metronome and thinks in polyrhythms, and then Tony MacAlpine and the other guy (forgot his name) are just great guitarists.
You can't forget King Crimson too, their music is so ridiculously complicated. Multiple time signatures at once, Robert Fripp with his midi layering, many other things all throughout their career.
Tonality: J.S. Bach
Technique: Ysaye or Beethoven
Rhythm: Igor Stravinsky

End of story.