R2-D2 and C3-PO
R2-D-2 and C3-PO are the only droids that were in every single epesode and the only ones they first met in tatooine the designer of C3-PO was Anakin Skywalker. They were always friends.C3-PO could translate any language. R2-D2 could send messages were ever they were. They survived till the end.