Two of my all-time top 10. Carmen is undoubtedly holding up better, but Pam is peerless in the entire glorious history of my knob going stiff, so I'd go with her.
Just realised that if I was answering this question objectively I could make a stronger case for Carmen, but back in the 90s Pam and me had an almost exclusive masturbatory relationship at times (obviously that relationship was entirely one-way). Because of that, Pam gets my vote.