My top 10:
1) Rod Laver
2) Bjorn Borg
3) Roger Federer
4) Pete Sampras & Rafael Nadal (Nadal is the more complete player but Pete has more slams)
5) Boris Becker
6) John McEnroe
7) Ivan Lendl
8) Andre Agassi
9) Jimmy Connors
10) Mats Wilander
Apart from the much older players, I decided on the basis of who would win at the peak of their powers.