I'm surprised no one's mentioned the other prestige classes in the DMG. The Dwarven Defender and Loremaster are pretty good.
If you'd like some advice, design your own prestige classes that are custom-fit for the campaign. That's really what prestige classes are for: to add flavor and depth to your setting by being specific about the culture and people living in your world.
Also, the Duelist/monk won't be insane unless you're letting people play with 35 or 40 point characters, or else characters with very strange ability scores. Remember that D&D 3rd Edition is designed for use with 25-point characters with only one or two scores above 14. If you're letting people play with a 16 in dex, wis, and int, that's already a 30-point character if they have 8s in everything else.
How good will the duelist/monk be with the default array? That's 15, 14, 13, 12, 10, 8.
Obviously, the 15 goes in dex and the 14 into wis, leaving a 13 for int. He's probably gotten 8 levels by now so that's a 16 dex, 14 wis, 14 int. He also gets a +1 for being a monk and likely +2 or +3 for magic items and such.
So that's an AC of 21, and its also great because most of it applies to touch attacks, and the flat-footed is okay too. Remarkable for someone who doesn't wear armor, but for a 10th level character (or so), it's not that impressive.
Consider that a fighter, paladin, or cleric of about 10th level can have an AC of 23 without too much effort: Dex 12, Full Plate +1, Large shield +1. Granted, his movement suffers for it and his touch AC will be nothing at all, but this is with very little material investment and the good ability scores saved for other things.
Just my two cents.
-Shurai