Griffon Merc: Questionable, low amount of slots make for extreme vulnerability to spell attacks. Lack of saves. Bonus: Several really good material griffon-only weapons out there. Better off going MR barb - don't have to worry about the saves. Main weakness of no flight for MR barb mitigated by race.
Sprite Merc: From what I've seen, pretty good.
Giant Hellion/Paladin: Paladin? No, you need spells+skills to make the class work, giant doesn't have the mental stats. Hellion, only if you plan on being purely melee (focusing on cleave) otherwise you're giving up a lot of useful hellion spells. I wouldn't do it. I'd rather play a minotaur hellion even with all their weaknesses than a giant (even though you can't because of stupid alignments
)
Sprite priest is good IMO.
Halfing sorcerer, not high enough INT. *Unless you depend on certain cabal spells*
Giant rogue? Not sure, I've seen a few but they always sort of disappear and I never hear about them again.