2006 is my vote because he was so dominant from beginning to the end of the year. If people wants to poke hole on Federer, pretty much you can do just about every players that had an excellent year.
Laver in 1969 was great, but he lost 16 times, which his winning % wasn't in the 90, unlike some of the other players(ie Fed, Borg, Mac). Laver have said the GS today is worth 2 GS in his heyday because the standard is a lot higher. And he won those 4 slams on 2 surfaces. Based on these facts, Fed 2006 > Laver 1969.
Nadal won 3 slams in 2010, but the problem is he only won 7 titles, no WTF, and winning % is below 90.
Nole had the best run in history from the start of the year to the USO. But his problem is he was a no show after the USO. Basically, if he didn't play at all after the USO it wouldn't make any difference, only to have a higher winning % but that would be due to lack in number of matches played.
Mac 1984 will be remember as having the highest winning %. But he only bagged in 2 slams. And since Fed also had a great year in 2005 with 2 slams, I don't think his 1 additional loss(81-4) means Mac 84 leapfrog Fed 05.
Tier1(goat): Federer; Tier 1.5: Laver, Sampras, Nadal, Borg;
Tier 2: Lendl, Connors, Rosewall; Tier 2.5: McEnroe, Agassi