Moss/Welker. No Contest. As @DeusIrae put it, one of the greatest WR's ever and probably the greatest slot Receiver ever. Fryar and Margan were two incredible competitors who were a critical part of late 80's Pats, but not in that class.
Just to be sure I wasn't missing something, I went to the stats:
Moss and Welker overlapped for three seasons (07--09). Together they accounted for 7,453 Yards Receiving and 62 TD.s
Fryar and Morgan overlapped for four seasons from 87--90, But the numbers don't even compare: 4,474 Yards Receiving and 31 TD's.