Fun topic. I saw Rice in all his years and he was just...insanely good. But then again, so was Moss.
It's really hard to determine the best 5 years of these guys' careers, because they both had so many great ones. Here's my best shot at pointing out numbers at these guys' respective peaks:
Rice
1989: 16 g, 82 rec, 1483 yds, 18.1 ypc, 17 td, 18 AV
1995: 16 g, 122 rec, 1848 yds, 15.1 ypc, 15 td, 19 AV
1987: 12 g, 65 rec, 1078 yds, 16.6 ypc, 22 td, 14 AV
1993: 16 g, 98 rec, 1503 yds, 15.3 ypc, 15 td, 20 AV
1990: 16 g, 100 rec, 1502 yds, 15.0 ypc, 13 td, 13 AV
Moss
2007: 16 g, 98 rec, 1493 yds, 15.2 ypc, 23 td, 20 AV
2003: 16 g, 111 rec, 1632 yds, 14.7 ypc, 17 td, 17 AV
1998: 16 g, 69 rec, 1313 yds, 19.0 ypc, 17 td, 17 AV
2000: 16 g, 77 rec, 1437 yds, 18.7 ypc, 15 td, 17 AV
1999: 16 g, 80 rec, 1413 yds, 17.7 ypc, 11 td, 15 AV
Hard to say which prime was better statistically. I guess I'd say Rice. But now look and see who the QBs were that were throwing to them in those years:
Rice
1989: Joe Montana (33 yo)
1995: Steve Young (34 yo)
1987: Joe Montana (31 yo)
1993: Steve Young (32 yo)
1990: Joe Montana (34 yo)
Moss
2007: Tom Brady (30 yo)
2003: Daunte Culpepper (26 yo)
1998: Randall Cunningham (35 yo)
2000: Daunte Culpepper (23 yo)
1999: Randall Cunningham (36 yo), Jeff George (32 yo)
Of their peak years, Rice had two HOF quarterbacks throwing him the ball, but Moss had the GOAT plus two guys who did make pro bowls, but weren't HOF level quarterbacks.
Here's one thing we should consider. Culpepper came into the NFL and had Randy Moss as his #1 WR. He put up three unbelievable seasons out of five during the time Moss was there. In 2004, Moss' last year with Minnesota, Culpepper had this passing stat line:
379-548 (69.2%), 4747 yds, 39 td, 11 int, 110.9 rating
And then in 2005, after Moss left, here was Culpepper's stat line:
139-216 (64.4%), 1564 yds, 6 td, 12 int, 72.0 rating
Now Culpepper only played 7 games that year. But for the rest of his career (which was injury-plagued), without Moss to throw to, he put up a combined 59% completion, 20 td, and 32 interceptions, with a 71.7 rating.
It seems like Moss made a HUGE difference in Culpepper's career, while Montana and Young were successful anyway.