Underrated currently: Giroud. Constantly getting flak and yes he does miss a fair few types of chances but he's great at build-up play.
Underrated in the past: Kanu. When people talk about Arsenal's recent great strikers they usually just name Henry, Bergkamp, and Van Persie, and he did a decent job in his short time here. Plus he won pretty much everything.
Overrated currently: It's tough but I'd have to say Bellerin... I can't say too much in support of him lately other than "he's fast"
Overrated in the past: I don't have an immediate standout for this so I'll just plump for Gilberto Silva. Seems to be held in the same regard as Vieira by some fans and I can't remember anything special. But then I was a kid, so...