According to the original novelization of Star Wars: A New Hope, credited to George Lucas: Both R2-D2 and C-3PO.
Attracted by the commotion, Solo looked back over his shoulder, frowning slightly. “Let him have it. Your friend’s way ahead anyhow. It’s not wise to upset a Wookiee.”
“I can sympathize with that opinion, sir,” Threepio countered, “but there is principle at stake here. There are certain standards any sentient creature must hold to. If one compromises them for any reason, including intimidation, then one is abrogating his right to be called intelligent.”
“I hope you’ll both remember that,” Solo advised him, “when Chewbacca is pulling the arms off you and your little friend.”
“Besides that, however,” Threepio continued without missing a beat, “being greedy or taking advantage of someone in a weakened position is a clear sign of poor sportsmanship.”
That elicited a beep of outrage from Artoo, and the two robots were soon engaged in violent electronic argument while Chewbacca continued jabbering at each in turn, occasionally waving at them through the translucent pieces waiting patiently on the board.
- Star Wars - Episode IV: A New Hope, George Lucas (ghost written by Alan Dean Foster)
This confirmed in Legends, specifically in the formerly canonical novel Shadows of the Empire. Chewbacca and C-3PO are playing the holo game when this passage takes place:
Chewie said something that sounded angry and impolite.
Threepio said, “Well, I’m sorry, but it was a fair move. It’s not my fault you didn’t see it.”
Chewie said something else.
“No. I’m not going to take it back. And don’t threaten me. If you pull my arm off, I won’t play with you anymore.”
Chewie muttered something, then leaned back on his seat and looked at the game board.
Note: I don't like answering my own questions, much less accepting my own answers, but I just stumbled upon this and had to post it as an answer, since it is effectively Word of God. In order to prevent myself from gaining rep from this answer, I've made it Community Wiki.