I'm going to be totally, completely biased here:
If it were me, I'd want it to be Golthagga.
Why? Because smiths are cool. Smiths tap into a very subtle kind of magic, as opposed to the "whapow, fireball, haste, chain lightning" of D&D wizards. They're symbols of strength, of skill, of the ability to take raw materials and turn them into something completely different. Evil supernatural smiths are fantastic — they can give rise to clockwork monstrosities, animated suits of thorned armor that bleed molten brass, any number of cursed or sentient (or both) weapons that you like. They can be an excuse to bring more magical materials into the world — forget mithral and adamantine, what about something like Planescape's green steel, or a magic-sucking black iron that is death not only to fey, but to magical creatures of all sorts? Metal and fire are as evocative as you can get. I say use them.
Golthagga takes the smith archetype one step further: He can forge flesh and bone like metal, and his followers might be able to do the same. Think of a half-fiend that's the result not of human-fiend breeding, but of a fire giant druid/smith of Golthagga placing a mortal and a fiend on the same anvil and hammering them together into some freakish flesh alloy creature. Any template can be used in this way. Fire elemental creature? Sure, it's had fire forged into it on Golthagga's anvil. Half-plant? If the template's out there, this is as good a reason to use it as any. One of those half-iron golems from the Monster Manual II? I don't think I have to draw you a picture. The sort of antagonists the PCs would be up against in a Golthagga epic, the sort of magic items they'd find... gives me shivers to think about how much fun it would be.
Any chump can destroy. It's much harder to create. To forge requires strength and skill and the will to make things the way that you want them to be. And the thought of an epic archvillain whose sole purpose is to create, but who turns everything he touches into something wrong and unnatural through the corrupt nature of his forge — a forge that should, by rights, be an emblem of purity! — that's pretty damn cool, the way I see it.
(This is also the reason I like Corean more than most LG paladin-supportive gods; he's also a god of the forge, with all the damn cool imagery that entails.)
But that's just me, and I'm biased much.
Chern get's my vote.
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