I vote the maze before reaching Sarevok's lair in BG1 as the worst tunnels of all time.
But I always like the maze in Spellhold. Some fighting, some cool riddles, some drama. I also like the tunnel fighting in the Witcher, since they're pitch black and even with a Cat potion one can't see more than two metres ahead... makes for some great atmosphere combined with ghosts, graveirs & vampires.