(includes BoVD stuff)
Human Wizard 5, Ur-Priest 2, Mystic Theurge 10, Archmage 3 - typical evil wizard type with the added flexability of being able to toss in cleric spells if you want to throw your players a curve-ball.
Tiefling Ranger 5 (FE: Human +2, Celestials +1), Mortal Hunter 10. 5 levels to play with. Bitter hunter-of-man type.
Human Fallen Paladin 5, Blackguard 5, Disciple of Dispater 10 - dark warlord type, can also have fun with Arterial Strike and Hamstring feats as they have the requisite ammount of sneak attack damage.
Half-Fiend (I perfer w/o wings) Human Bard 7, Spelldancer 5. Good cohort character - have the bard spells focus on weakening, disabling, charming or cursing your opponents. Bestow Curse, Dirge, etc
Human (did I mention I like humans?) w/ Shadow-Walker template Rogue 7, Shadowdancer 7, Flammar Shadowlord 5. Lord of Darkness, untouchable sneakster character. Can more or less get into and out from just about anywhere.
Former-human Ghast (replace 'stench' with 'aura of oppressive evil' descriptor .. identical mechanic, though) w/ Spellstiched template and as many levels of monk as you can get away with. Going by straight CR, Ghast is CR 3, Spellstiched is +1 Cr, and 16 levels of monk would total out to CR 20. Have a wisdom over 19 (good for being a monk anyway). Suggested spellstiched abilities: (1st lvl): Mage Armor 1/day, Divine Favor 3/day. (2nd lvl): Desecrate 1/day, Death Knell 1/day. (3rd lvl): Blacklight 1/day, Negative Energy Burst 1/day. (4th lvl): Divine Power 1/day, Enervation 1/day. (5th lvl): Wall of Force 1/day, Unhallow 1/day. (6th lvl): Harm 1/day. Invest in cheesy monk-only magic items such as Ki Straps, Monk's Tattoo, Monk's Belt (leave out if you're leery of haste items), and Training Dummy of the Master. Possibly also invest in an Item of Disguise (as the Hat) to be able to conceal his undead nature... or just have him embalmed, etc so he was turned undead before he decomposed.
Former-elven Deathknight (heh, no con means no con penality!) Fighter 2, Monk 1, Ranger 1, Bard 2, Bladesinger 10. 4 levels to play with. Funny really, I find myself liking the idea of a fallen bladesinger much more than a normal one. Has Int, Wis, and Dex mod to Ac while unarmored, Int and Dex while wearing armor. Normally wearing armor incurs ASF% on monk special abilities, but heh, a bladesinger of sufficient level ignores ASF in light armor. Which would include something like a Mithril Breatplate.
Hope this helps some.
Edit/Additional: Oh yeah - and have the spellstiched ghast monk invest in a wisdom enhancement item. possible also a book of +wisdom too if he can swing it.