Most beautiful to watch, stylistically? Most graceful? Most creative? These are all things. They can't be measured in numbers though. When that's allied to 20 grand slams, I think it's perfectly viable to argue for Federer over the others.
Why does there have to be a greatest at all ? Short of someone going out and winning 50 grand slams surely you can't call it. It's a modern obsession I think. I remember Borg being amazing, like a leech, you couldn't shake him off. Ashe was classy. Edberg graceful. McEnroe petulant, irritating, genius. Connors relentless. Becker a wrecking ball. Sampras boring, Agassi fantastic. There were great characters and brilliant tennis players.
I've dropped off in my interest a lot over the last 20 years but Federer, Nadal, Murray, Djokovic absolutely brilliant players. There is no GOAT for me. They all have fantastic talent and mental strength. That's why I'm happy Djokovic won on Sunday, it's truly an amazing story what he has had to carry and what has been lined up against him before he even got on court. I've never seen so much spite, vitriol and hate put against any tennis player. Like him or not ( and I never had any particular feelings about him) it's some victory. I don't think (and I hope) I will never see anything like it again.
But does that make him the greatest tennis player of all time ? No. You can't judge players of different eras against each other. For you guys and girls who love tennis this last 20 years must be a brilliant epic era. 4 great players battling it out. They all deserve great respect.