Since when were monks overpowered or broken? I think DM's ban monks more often because they just don't like Shaolin, bare-fisted warriors standing toe-to-toe with Joe Fighter and Bob Paladin. Also, looking at what they can too, it's clear to see that this "broken" quality is more myth than anything else. There's more than one way to kill a monk.
Class balance is interesting. There's even spreadsheets out there done by people that kind of "calculates" them in a point-rating system. But, those ratings also calculate based on the assumption that everything is orderly and fair and equal, but I think for the most part are pretty accurate.
For instance. The Cleric is quite a powerful class if utilized in certain ways. And can often, at higher levels, become stronger than other classes in other niches, if they are given time to buff up. However, the Cleric does have a check and balance against it. D&D also kind of "assumes" that you'll be roleplaying, and as such a DM could easily control characters that rely on deities or Forces of Nature for powers. The Cleric is only overly strong if you throw out peripheral rules, like hoping a Cleric actually behaves like a cleric and not just a warrior with extraordinary powers.
The druid can also do some pretty nasty things if you min/max him properly, and use abusive strategies. Just look around this board or the WotC optimization boards. They can, at any level, prove to pretty sadistic against fighters. However... again, D&D assumes that when you play a druid that you'll actually play a druid... and not just a warrior with Powers of Nature that he freely abuses to kill people with fire seeds.
Finally, I would also put monk at the bottom of the list. A high level monk has about one thing going for him that makes him "awesome". And that is his SR. Otherwise, he's no more special than any other class. His magic items don't make him impressive because any fighter or rogue worth his salt is going to enchant his equipment with the same stuff. He can't trip better than a fighter. He can't hit better than a ranger. His powers are one-hit wonder attacks. The monk, in some cases, if you follow the rules, is lackluster (unless of course I once again throw out the concept of monk and min-max to my pleasure). I don't think any of the core classes are broken, least of all of them the monk.
