With my Arsenal cap on I'll say Bacary Sagna who whilst universally recognised as 'good' doesn't get enough praise. There is not a single better right-back than him in the world.
Elsewhere I think Everton are litterred with underrated players: Fellaini is an incredible footballer and I don't know why he hasn't been snapped up. Cahill may not be gifted with pure talent but he is efficient and consistently scores crucial goals. I don't think Busquets is appreciated enough (as a player I mean, he's also a cheating ****) and other Premier League players like Kompany, Gyan, Ivanovic and N'Zogbia are pretty underrated.