Monk.
The problem with the monk is that he isn't really a party-building character. The monk's strongest suit is mage-killing, but when you examine *why* he is good at mage-killing, his use to the party becomes minimal. The reason that the monk is good at mage-killing is because of his DEFENSIVE capabilities, not his offensive ones. Indeed, in terms of offense capabilities, the monk has perhaps the weakest in the game- primary fighting classes (barb, ftr, pal, rgr) can simply dish out more damage, rogues can trump if they can sneak attack and all of the primary spellcasters can easily dish out more damage.
Their defensive capabilities make them great for one-on-one mage-killing, but really suck at multiple enemy battles. The mage simply ignores the monk and blasts the rest of the party, soaking up the damage that the monk can hit him with. Even Stunning Fist, the monk's only real effective attack against a mage, is superseded by even low-level spells.
Solo, monk is a nice class. In terms of party games, it's all but pointless.