Each of the top 4 have a seperate favourite slam to play
The following list isn't a list of who is best at each slam, but rather asking each of the top 4 what their most favourite Slam would be to play - what would they answer. My guess is the following. Interesting that they have one each.
Australian Open - Djokovic
French Open - Nadal
Wimbledon - Federer
US Open - Murray (though he must like the AO as well).