I find this lame for some reason. If you need another person to define if you are the GOAT or not, then you are not the GOAT. Federer is GOAT based soley on all of his achievements.
You find it lame because you're a massive Federer fan. All I'm saying is without Federer the man to beat would be Sampras who Nadal could probably top. I'm not saying it should be something people think about a lot, not something that should keep Nadal or his fans awake thinking "If only Federer hadn't set the bar so high" it's just an interesting idea that people don't give much thought because it is always assumed that Nadal robbed Federer of so much that if he didn't exist then Federer would gain so much, but Nadal would gain a big boost in status without Federer.
Agassi would have 14 slams without Sampras. Actually he might have about 18 as his 96-98 slump would have never happened without Sampras beating him at the 95 U.S Open. No player has suffered as much from another player as Agassi did from Sampras.
How did you arrive at 14 slams? You mean because Agassi lost 6 times in slams to Sampras? You're assuming every time that Agassi lost to Sampras he would have gone on to win the slam if he didn't play Sampras. That is really flawed logic, especially since not all of the matches he lost were even finals.
Let's look at each loss.
US Open 1990, ok Agassi would have probably beaten McEnroe who lost to Sampras in the semis, but Sampras also beat Lendl who would have had a great shot vs Agassi in the final (he beat him at the US Open in 1989 and had won the AO that year) If Lendl makes the final which he probably does, Agassi is the underdog.
Wimbledon 1993. Agassi lost in the quarters, yet he's a sure bet for the title? (with all those grass greats around

) There's Becker, Courier, and Edberg. Becker lost in a 5th set to Agassi in 1992 but beat him in 4 in 1995. No sure thing for Agassi at all and I maintain he was far from a great grass court player.
US Open 1995. This one I see Agassi winning, don't think anyone else would have beaten him here.
Wimbledon 1999. Late in the 90s there wasn't much competition around at Wimbledon and Henman would likely have made the final in which case Agassi wins.
US Open. 2001. Playing back to back against Safin and Hewitt, who knows? 50/50 chance.
US Open 2002. Pete had a pretty easy draw so no-one he beat would have really been that dangerous. Roddick was too young, Haas may have beaten Agassi if he got to the final but most likley Agassi wins this one.
So out of that I say 3 more slams, 4 tops. 12 slams with the full set might put him where Sampras is at the moment.
Also no player has suffered as much from one other as Agassi from Sampras? Erm Fed and Nadal..