One of my personal favorites is actually a comic I just recently managed to grab ahold of: Detective Comics Annual #8. It's a retelling of The Riddler's origin, part of the 'Four Of A Kind' collection of origin annuals.
And personally, my favorite thing about The Riddler is not what the villain offers himself, but the keen detective skills that he brings out in Batman. Whereas The Joker and Two-Face usually require brute force and/or reasoning to be contained, The Riddler makes it all a mindgame. He's absolutely and utterly obsessed with playing a game of chance with the lives of every citizen in Gotham City, just waiting for that day when Batman won't be able to outsmart him. But given that he always does, The Riddler is compelled to keep being who he is. It really plays on the whole debate of whether or not Batman's villains would exist without him, as The Riddler more directly exists to challenge him alone. Batman is his ultimate adversary, which is what he was looking for from day one.
To answer your question, though, I think it's the fact that The Riddler is so smart that makes him a danger to Gotham City. While The Joker is a criminal mastermind, he doesn't exactly make it a point to make every last one of his crimes focused on that point. Two-Face is too conflicted by his psyche and his dependence on his coin to really use all of the mental potential at his disposal, which usually leads more to violence than feats of genius. But The Riddler is a villain that does rely on his genius, which leads to lethal puzzles and deathtraps that no one else could invent or even begin to understand. Batman's the only one that's managed to figure him out.