You have to look at Borg and Johnny Mac in their primes. Borg in those Frenches (78 and 80) without losing a set was huge. Mac in the 83 and 84 Wimbledons was untouchable. Federer's Aussie Open triumph this year (07) qualifies - didn't lose a set and completely demolished Roddick.
Borg also didn't lose a set at the 1976 Wimbledon championships.
I'm not sure about pre-open era. I hear Lew Hoad in his prime was insanely good.