I am set on a warrior, but was wondering if a shaman or a paladin would be a better second character.
Don't think you could go wrong with either of those honestly if you're talking about PvE. If you weren't talking about potential healers it would be a different story. Both Pally & Shaman bring excellent group buffs as well as versatility.

Personally - I'd take the pally as the buffs are more of the set & forget style vs. the totems which you'll have to drop & recall constantly.