The protagonist's story usually. That would be Rey and Finn, so far. Kylo Ren is not the protagonist of the story (so far). Poe might gain more as we go on. These stories are likely not Leia or Luke's story, as they are supporting characters.
The PT that is basically Obi-wan, Padme, and Anakin (aside from The Phantom Menace...where it is difficult to find the protagonist, since it seems to be Qui-Gon and Padme, then Anakin pops up, and Obi-wan is sidelined for half the film). Anakin, even when he falls, is still the protagonist of the story as we watch the hero fall to darkness.
The OT it is Luke mainly, with Leia and Han. Darth Vader is not the protagonist at all.