The worst NFL stadium I have ever been to is Texas Stadium, hands down. That place is a complete dump. And the frickin' hole in the roof is just stupid.
Of course, I have only been to Arrowhead, the Meadowlands, FedEx Field (or whatever it's called now) and the Ralph for football games. I've been to the Stick, the Oakland Coliseum, the Vet, Three Rivers, RFK, the old Cleveland Stadium, the Kingdome and Busch for baseball though.