I'd probably say Greg Ellis. A lot of fans will never give him his due because they're still bitter we drafted him over Randy Moss. I'm not defending that, Moss is one of the greatest talents to ever grace the NFL and we SHOULD have drafted him(and so should have the other 17 teams that passed on him), but Ellis was a very good base End for a long time and a key part to some excellent defenses in the early 2000's... especially after we added Glover up front to give him some help. It's just a shame those defenses will be forgotten because of how inept we were on the offensive side, it was pitiful. Coakley, Ngyuen, etc.... all in that same boat.
He'll never be a ROH guy, but I'd consider him in that next group of Cowboys that were key players during their tenure.
Though I have to say, his off-season comments about being misused during his last few seasons also have a hand in how he became under-appreciated by fans. Definitely rubbed some people the wrong way.
But mostly... He'll forever be linked to Moss, and that's the connection that will always plague his legacy.