Some of my thoughts....
John Salako - The guy was a major catalyst in our promotion season and stabilising our championship position. We've been lucky to have some good wingers in the last few years but Sally always seems lost behind a cloud of Little, Shunt and Convey.
Kevin Watson - Again a major catalyst for promotion and beyond did his job in midfield well and was a nice bloke to boot.
Keith McPherson - Again rock solid defender for nearly a decade but got lost behind the likes of Dariuz, Adie Williams and Linvoy Primus. The guy played at Wembley with a broken foot ffs

Martin Williams - Whilst 'skittles' was never a great player he was one source of entertainment through a couple of dour seasons at the Madejski.
Ulises de la Cruz - As mentioned in the header, whenever the Premier League years are mentioned this guy never gets a mention. He was never brilliant but rarely awful and was a nice part of the club for a couple of seasons.
Any other players over the years who you thought did well but never got the credit or attention they deserved?