I think without any bias, the answer for everyone should be indoor hardcourt. That controls all the variables. The weather, the lighting, the court surface are all controlled. It's just two players playing the game. Except for the crowd who can be unpredictable.
But some players prefer the chaos, and the chaos can honestly be a leveling factor. Wimbledon, Roland Garros, USO, and the AO are all chaotic to some extent. You never know what'll happen. And that's more true to real life. It's a bit more compelling, because in our every day lives we know that all the variables aren't controlled.