Well you want something that's reasonably hard to kill, and that there are a lot of. Vampires is a good idea, though if you've got Minsc his enemy is vampires, so you mightn't want to double up. I don't think you can get beholders, but that would be good too. Demons maybe? There are quite a few, or golems (but again I don't think you can choose golems). Dragons maybe, but there aren't that many and it probably wouldn't really make a difference, if you're not tough enough to kill a dragon, having it as a racial enemy probably wouldn't help, same with something like a lich. Oh also mindflayers, there are heaps of those.
So my picks are vampires, then mindflayers, then demons (unless you can pick beholders etc)
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