Clay. Nothing is as good as a 3-hour-French-Open fivesetter for me.
On today's slowass surfaces you can't have a 3-hour 5-setter even on grass.
To this thread's question, I like grass and fast HCs the most. Those are the surfaces favouring offensive tennis. Or in this slowass era, they're the surfaces punishing offensive players with poor defensive abilites least.
On the other hand, two talented players on clay may be cool, like Stan-Richie at RG this year. You have some longer rallies with nice shotmaking but it isn't an endurace battle like between two pushers.