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I think there is a reason why D&D is the most popular tabletop RPG.
So if it's about game mechanics, I'll chose Temple of elemental Evil or Neverwinter Nights, who have a more modern and complex sistem, depending if you prefer turn based or real time combat.
Temple of Elemental Evil is really hard to learn to play, more if you don't know the 3.5 edition of D&D, though, but is incridibly accurate to the tabletop game.