Coolest 1 lvl dip for fun abilities!

[sirpercival]

LA +1 for Vecna-blooded?

Actually, the hilarious thing is that if you gain the template and then lose it, you keep Cloak of Mystery and lose the LA +1.  So that won't even give you the extra level.

[Lycanthromancer]

LA +1 for Vecna-blooded?

Actually, the hilarious thing is that if you gain the template and then lose it, you keep Cloak of Mystery and lose the LA +1.  So that won't even give you the extra level.

Also, according to Savage Species I can get any template I want with Wish. My Spellcraft is definitely high enough, fo' sho'.

[CattyNebulart]

Well, there are some fun soulmelds from here.

Other than that, I think you've covered all your bases.  You can get pretty much any ability you want, so there's not a whole lot that 1 level of something is going to get you...

Just so you know, I like abilities that change the rules, such as the way spellwarp sniper alters spells to rays (though one dip only affects low level spells), and how geomancer lets you cast arcane as divine and vice versa.

I thought maybe exemplar, to take 10 on every skill in the game (and then some), but my skill mods range to several hundred (each).

Hmm...

Well then how about truenamer? With that high a mod you should be able to use it 

[Lycanthromancer]

Well, there are some fun soulmelds from here.

Other than that, I think you've covered all your bases.  You can get pretty much any ability you want, so there's not a whole lot that 1 level of something is going to get you...

Just so you know, I like abilities that change the rules, such as the way spellwarp sniper alters spells to rays (though one dip only affects low level spells), and how geomancer lets you cast arcane as divine and vice versa.

I thought maybe exemplar, to take 10 on every skill in the game (and then some), but my skill mods range to several hundred (each).

Hmm...

Well then how about truenamer? With that high a mod you should be able to use it 

I'll have to go with that. Because +1 to AC at level 20 is obviously AWESOME.

[awaken_D_M_golem]

  ... or Half-Fey 1 so that once a day the Charm + Hypnosis
target can tell you with all their heart, that you have +1 to AC
and are obviously awesome ; and believe it forever and ever.

[sirpercival]

OK, I think I have it:  Fiend of Possession.

Because with Greater Spell Immunity (Holy Word, Dismissal, Banishment, Exorcism) and a huge Hide check, you're essentially undetectable if you want to be.  You can only hide in objects, but that's okay -- make yourself an awesome object with all the Haunt Shift tricks and then animate it with TK or just with Animate Object.

[Rags]

You said that you have allot of feats available, but could you do anything with the Eidolon/Eidoloncer from Ghostwalk? If not, the 1st level of the Ghost Slayer PrC, in the same book, grants Immunity to Possession. Kinda niche, but I cant find it anywhere else (didn't really look...).

In fact, have a look at Dark Scholar from Shattered Gates of Slaughterdarge. 1 level can grab you either the ability to cast spells 2 levels lower then your max without verbal components, use divine scrolls as if you were a cleric of your arcane caster level (guessing you have a big arcane caster level), or the ability to age at half your rate.
Also the Serene Guardians resonance ability is pretty cool.

[Prime32]

Binder maybe? Constructor or Body Leech?

[mthor]

Ninja or dread pirate. End the debate forever.

[Mister Lamp]

Acolyte of the Skin.
Pointless ability-wise? yes
Worth it? yes. with abilities like yours all you need is the skin of a demon.

Beloved of Valarian
Unicorn!

Drunken Master

[Yirrare]

I just realized the mindbender from complete arcane might be what you are looking for. Communicate telepathically with any creature within 100 feet that has a language. Neat!

Kindly
Yirrare

[RobbyPants]

Warlock or DFA would get you a least invocation, which could potentially be fun depending on which one you pick.  I'm not even going to pretend a 1d6 Eldritch Blast would be useful to this character, but breath weapons are always cool. 

At will Shatter is kind of nice, although if it always has a CL of 1, you can't affect things that are too heavy.

[Lycanthromancer]

Warlock or DFA would get you a least invocation, which could potentially be fun depending on which one you pick.  I'm not even going to pretend a 1d6 Eldritch Blast would be useful to this character, but breath weapons are always cool. 

At will Shatter is kind of nice, although if it always has a CL of 1, you can't affect things that are too heavy.

Plus, my build has infinite power points (Font of Power + Temporal Reiteration = manifest all day every day at max potential), so I already have Shatter at will. And Summon Swarm. And pretty much any non-epic spell or power in the game.

I tweaked myself out with a Magic Mantle/Spell To Power society mind, and abused a thought bottle, Extra Spell, research, Psychic Chirurgery, and the erudite's learning mechanics on a better-than-erudite chassis. (Though it's only better because it doesn't have the UPPD limitation.)