A Rogue13/Ranger6/Monk1. An absolutely incredible build that is just now starting to get the credit it deserves, and will only become more powerful when the cap increase allows it to complete. Self-sufficient, uber-untouchable AC, godly saves, incredible to-hit, immunity to knockdown, sufficient DPS to hold raid boss agro, insane DPS with sneak attack enabled. Great ranged damage in a pinch (manyshot)... all at the same time.
There are many variations, some go 2 monk, some go more ranger, etc... but this is the one I love the most. It's just plain sick. Only problem is that some groups refuse to believe a rogue can do the things you volunteer to do (take down titan HP, hero Sulu, hold ortho agro and kill two of them before the group finishes the one they're working on
).
It's a rich man's build, you have to be able to farm/buy the equipment and have the tomes for it... but when you're equipped, you're nearly invincible.