Unfortunately in this homogeneous court era, winning the 4 slams are not that much of a big deal. Big deal is what Agassi did, in a time where the courts of each slam were radically different
Beijing Olympics win -> Very fast hardcourt
US Open win -> fast hardcourt (before it was slowed down ridiculously in 2011)
Wimbledon wins -> grass, it's grass!! still acts like it and you're required to play really different to other surfaces
RG wins -> Nothing needs to be said, even did it when they fucked up the court/balls in 2011 so that Olderer could win it again...or so they thought
AO win -> Medium hardcourt is a surface you need to be able to play on as well, you can't be diverse if that means 'fast courts only and a clay one'
And he won just about all his titles by beating your precious boy Federer along the way