In terms of slams they're all going to be close. And as others have said it's about personal taste, but for me Federer is the best all round player I've ever seen. And I'd put Rafa 2nd, they are more WOW than Novak whom I respect so much. He's the best competitor I've ever seen in any sport, a phenomenon and is so hard to get past, and his return is on another level. But Federer is an artist and represents what I love in sport be it football or tennis.
I remember a game years ago and the commentator said its like your witnessing greatness. I remember the first time I saw him in Wimbledon in his first win and you could tell this was a different player to anything we'd seen before. And I was a fan of Roddick back then so was cheering him on. Everything was laser like accuracy and the movement from side to side was like he was gliding on air. The whole stadium sweating in the sun and the only one not was Roger, pure grace pure sporting art. For me the GOAT for those reasons even if he has a small amount of majors fewer than the other two.