Federer is the most talented of the two, but in terms of 'greatest' and 'achievements', then Sampras is the only contender:
Level of opposition, stats, Slams won, titles won, etc:
Pete Sampras
Level of opposition: Andre Agassi** (20-14); Jim Courier** (16-4); Goran Ivanisevic (12-6); Boris Becker** (12-7); Michael Chang (12-7) Patrick Rafter (12-4) **denotes multi-Slam winner
Slams: 14
AMS Masters: 11
Masters Cup: 5
Singles: 34
No.1 for 286 weeks
[f]Finished[/b] at No.1 for 6 years running
Best year stats: 1994 (77-12: 2 Slams; 8 ATP titles)
Roger Federer
Level of opposition: Marat Safin** (7-2); Andy Roddick (10-1); Lleyton Hewitt** (11-7); Andre Agassi** (8-3)
Slams: 7
AMS Masters: 10
Masters Cup: 2
Singles: 19
No.1 for 120 weeks
[f]Will finish[/b] at No.1 for 3 years
Best year: 2004 (77-6: 3 Slams; 8 ATP titles)
Level of opposition, stats, Slams won, titles won, etc:
Pete Sampras
Level of opposition: Andre Agassi** (20-14); Jim Courier** (16-4); Goran Ivanisevic (12-6); Boris Becker** (12-7); Michael Chang (12-7) Patrick Rafter (12-4) **denotes multi-Slam winner
Slams: 14
AMS Masters: 11
Masters Cup: 5
Singles: 34
No.1 for 286 weeks
[f]Finished[/b] at No.1 for 6 years running
Best year stats: 1994 (77-12: 2 Slams; 8 ATP titles)
Roger Federer
Level of opposition: Marat Safin** (7-2); Andy Roddick (10-1); Lleyton Hewitt** (11-7); Andre Agassi** (8-3)
Slams: 7
AMS Masters: 10
Masters Cup: 2
Singles: 19
No.1 for 120 weeks
[f]Will finish[/b] at No.1 for 3 years
Best year: 2004 (77-6: 3 Slams; 8 ATP titles)