I see a strong case for Borg over Mac for peak. And if we are talking about an extended peak, then it's got to be Borg.
I admit that since McEnroe is my all-time favorite player, that I'm likely a bit biased. And man, did I love watching him on live TV back then; especially in 1984. 1984 was insane.
Among Federer, Sampras, Djokovic, and Borg, here are some interesting grass-court stats while playing against the best players
Most wins vs top 5
Federer 11-6, .647
Borg 7-2, .779
Sampras, 6-0, 1.000
Djokovic, 6-5, .545
Most wins vs top 10
Federer 21-11, .656
Djokovic 14-8, .636
Borg 13-4, .765
Sampras 11-3, 786
Grass-court stats vs top players through their age-25 seasons(excluding Borg, since those stats are already listed for him above)
Wins vs top 5
Sampras 5-0, 1.000
Federer 5-1, .833
DJokovic 1-4, .200
Wins vs top 10
Federer 9-3, .750
Sampras 8-2, .800
Djokovic 4-6, .400
Arrrgh. I might need to switch my vote to Borg. Mac has the HTH matchup advantage over Borg. But Borg crushed the field. What's interesting is this: Mac faced Connors on grass 5 times from 1982-1984. Mac was world #1 in 5 of those matches, yet Connors still went 3-3 against him. Borg was 4-0 on grass vs Connors from 1977-1981, which includes beating Connors in a Wimbledon final while Jimmy was world #1. Jimmy was world #1 in two grass-court matches vs Mac for his career and Jimmy won both of those matches.
The numbers clearly point to Borg over Mac for peak. My potentially biased eyes point towards 1984 Mac. I might have just proven myself wrong on TTW, a not-so-proud moment.