I always thought the Piper v Rude was pretty good feud. I'm maybe a bit young to remember all of it though. In terms of other memorable feuds you can't ever ignore Earthquake v Jake The Snake. That was a bit nasty with the snake killing and all. Mountie v Bossman was definately a highlight but my favourite was Hogan v Slaughter. I'm not even a Hogan fan but it was really good.
Additional Comment:
Actually thinking about real feuds you can't go beyond WWE v WCW.