It really depends on what you're doing and everyone is going to ride differently.
I mean you could technically full seat and two point every gait if you wanted too. When I sit the trot, I am doing a full seat and I have done a lot of two point at every gait, even the walk, to help build up strength. When I'm doing jumpers I prefer to do a full seat at the canter as opposed to a half seat, but if I were doing hunters then I would prefer a half seat over a full seat. Of course, if you're doing dressage than you would never do a half seat. As for two point, I really only do it over jumps and even then I'm someone who sits deep until the very last second because the horses I ride need that finer control.
So to answer your question... it depends. :)
The hardest part isn't finding what we need to be, it's being content with who we are.