If you post your build, it would be easier to give specific advice.
Lore Warden would be an solid choice, boosting the strenghts of the monk, without giving much that isn't useful.
While it might not fit your character, levels of Alchemist(Beastmorph Vivisectionist) might be rather useful. The mutagen itself is rather good for any martial character. The abilities of the Beastmorph archetype could be quite nice, but it takes some levels to be useful. The sneak attack of the vivisectionist should work nicely with the unbalancing counter ability. Additionally the Crane Style feats could help set up your AoOs.
Depends on your stats and what you want your character to do.
If you want to improve your combat ability probably a fighter or at least a full BAB class.
An Inquisitor would also work well with a monk. Adding bane to your flurry of blows in will be a nice boost to damage when you need it. Judgments will also be valuable for their versatility. Throw in some spells and you are good to go.
Fighter would be nice for feats and BAB. Styles!
Ninja for more ki powers.
Cleric for some use of your probably high wisdom.
A good PP that synergizes well with your Monk might be the best, but I can't think of which off the top of my head - without seeing your build at least.