Some of Rafa's wins in the RG final were some of the worst. Just last year he beat Ruud 6-3 6-3 6-0. And then Stan in 2017 6-2 6-3 6-1 and Ferrer in 2013, 6-3 6-2 6-3.
But yeah, the worst was 2008 RG: Rafa over Roger 6-1 6-3 6-0. That's the fewest games won by the loser in ANY Roland Garros of the Open Era.
Novak has had some lopsided wins at the AO, not as bad as RG 2008 but similar to the others. The worst recent one was Novak over Rafa in 2019, 6-3 6-2 6-3. Agassi over Schuettler in 2003 was worse, though: 6-2 6-2 6-1, and maybe the worst of the Open Era (Korda over Rios in '98 and Lendl over Mecir in '89 were close, both 6-2 6-2 6-2).
At the USO Roger over Hewitt in 2004 was pretty bad: 6-0 7-6 6-0. A tiebreaker sandwiched by two bagels! Edberg over Courier in 1991 and Lendl over Mecir in '86 were also bad: 6-2 6-4 6-0 and 6-4 6-2 6-0. But the absolute worst USO final was in 1974, when a young Connors beat an old Rosewall 6-1 6-0 6-1. Ouch. Maybe the worst Slam final ever.
Wimbledon: Roger over Cilic in 2017 was the worst in recent years: 6-3 6-1 6-4. Going back, McEnroe over Jimmy Connors in '84, 6-1 6-1 6-2, and over Chris Lewis in '83, 6-2 6-2 6-2 were worst. Borg over Connors in '78, 6-2 6-2 6-3, was also bad, as was Connors over Rosewall in '74, 6-1 6-1 6-4.