Starting with A New Hope I would say C-3PO really belongs to Princess Leia - he remains at the Yavin base with her during the battle, then in Empire Strikes Back he escapes with Han and Leia aboard the Millennium Falcon. As for R2-D2, he's pretty much Luke's as of the Yavin IV battle.
Interesting thing to consider though - C-3PO was built by Anakin but ends up with Padmé, while R2-D2 was originally owned by Padmé but ended up with Anakin. And then those ownerships kinda transferred over to Luke and Leia.
C-3PO's astromech sidekick in the Star Wars epic. One could argue that C-3PO is Luke Skywalker's sidekick, but according to Star Wars lore, Threepio had scores of masters, including Anakin Skywalker (if I recall correctly).