The kensai of course. Non of this wussie dual class to thief so they can wear armour rubbish! You want a pure level 40 kensai to dish out some massive damage per round
The fact that you can't wear armour isn't a problem, kensais should be used for flanking, not front line tanking. Regardleses, your weapon speed factor will very quickly reach 0, so you're guaranteed to strike first in a round, and strike hard too. Even the toughest monsters can't survive longer than a round with a high level kensai with grandmastery in spears and ** in 2-H weapon style using the Impailer +3. I had a level 40 solo one in the underdark (just capped the XP) with 21 STR, and his damage range was 42-47 each hit!!! Then of course, he gained a natural STR of 24 and got his hands of the Staff of the Ram +6 but that's another story.....
If you have a kensai in your party, its actually possible to reach an AC as low as:
Base AC: 10
DEX 18 (+4 AC): 6
Kensai bonus (+2 AC): 4
Ring of Protection + 3 (ToB only): 1
Ring of Gaxx (+2 AC): -1
Dusty Rose Ioun Stone (+1 AC): -2
Cloak of the Sewers (+1 AC): -3
** in single weapon style, or dual wielding 2 weapons that grant an AC bonus (e.g. Dakkon Zerth's Blade + Yamato +4) giving a +2 AC bonus: -5
Then cast spirit armour to get a further +9 AC giving -14!!!
Not bad to say your running around in your boxers!