I tend to prefer wizards (wider toolbox, faster spell advancement) over sorcerors (more spells, bloodline powers) when it comes to full arcane casters, and clerics over oracles for the same reason. But none of the four classes is a _bad_ choice for this.
Positive energy channelling is amazingly powerful in a horror campaign, but the right spell selection can make an arcane caster just as useful, and maybe more so.
Divine casters are usually more effective at helping friends, arcane casters at hurting foes, but if you want to do something specific you can usually build a sorceror or oracle to do that thing very well.
What else can you tell us about what you like to play and what's allowed?
If it were me, I'd make an aasimar sorcerer with a celestial bloodline, a dhampir sorcerer with an undead bloodline, or either a elemental fire or a red (gold) dragon sorcerer of any race. Its no secret that the AP is filled with undead, so why not make a sorcerer that can handle them, one way or another?
Witch is a bit handicapped in undead-heavy stories by many hexes being mind-effecting. They still can be useful, however.
Undead bloodline sorceror has the advantage of being able to use mind-affecting spells on undead. You can dip one level of that to be able to use that ability with cleric or wizard spells too.
Hex Channeler Hedge Witch
or alternatively....
Hex Channeler Gravewalker Witch
Witch is a bit handicapped in undead-heavy stories by many hexes being mind-effecting. They still can be useful, however.
Undead bloodline sorceror has the advantage of being able to use mind-affecting spells on undead. You can dip one level of that to be able to use that ability with cleric or wizard spells too.
Funnily enough, Witch isn't as handicapped as one would think. There's actually a good bit of the AP that has non-undead enemies, especially come books 2,3 and 4. In fact, being solely undead-focused (while undeniably helpful at some points) will leave you with good chunks of the AP where you feel less than helpful. In fact, there's a lot in Book 2 that you'll love having hexes to use against, since they don't actually go up against Spell Resistance. Just to let you know.
I'm going to second the Gluttony wizard. Really cool RP potential, though you should not be good. Another option is to be a Battle cleric of Pharasma (I think that's the one) you get a really cool something midway through the campaign that only a battle cleric of Pharasma can really use to full effect, so If you happen to be one, that's great. A neat option would be to have a level or a few levels of holy vindicator as I think it's a great presteige class for a battle focused cleric, though I'd stop at level 8.
Of the two options I'd take the wizard though as you play enough melee characters and should branch out now that the opportunity has arisen.