Federer and Nadal are great because of their forehand.
Murray and Djokovic are great as well because they're good on both wings.
It's hard to discuss because if you say Federer's backhand is better than Nadal's, then Nadal's forehand is better than Fed's to cover for lack of backhand? vice versa.. etc.....
And I would say that when Nadal was playing, his forehand was better than Fed's, but he's out right now so hard to say "currently".