Favorite build I've actually made...
Probably Azoth, The Raven's Shadow from my sig. Rogue 1/ Monk 9/ Rogue +9/ Warblade 1. Thrown sai with sneak attack, skirmish, and iaijutsu focus all reliably activated at once, on top of a highly effective stealth/scouting build that's very difficult to actually target in combat. At higher levels, just attacking him provokes opportunity attacks that also benefit from SA and iaijutsu focus (and potentially Skirmish if it's during your turn). All practical optimization.
Second place probably goes to The Siege Tower, also in my sig. Abuses the Symbiotic Creature template to get... I think it was 22 thrown daggers per round by ECL 8? This one's more into the TO range.
I've also always wanted to do a build that uses Ballisteer with siege weapons. Flaming, phasing cannonballs/boulders aimed straight at the enemy's gunpowder reserves...