If you mean "underrated" as "producers who don't get enough attention for their work" then I do actually have a list.
AvaliaKasa (I think that's what she goes by mainly, I'm not sure) and she has fantastic originals.
rageyn has the most amazingly tuned Ruko I've ever heard, I'm serious.
Sarah Lynn. I would kill for her tuning and mixing skills.
lilycat130. Great with Engrish, plus tuning and mixing is pretty good.
AdyS. Just. Best. Engloid. Producer. EVER in my humble opinion.
@xseerx I'm biased because he's one of my best friends, but he is AMAZING at tuning and mixing, plus his art is to die for.