Honestly, I don't think either of them would make a good king, but if pressed, I'd have to say Claudius.
Claudius saw what he wanted (the kingship and Gertrude), and he acted to get it. Once he was king, he acted to solidify his power, but didn't act maliciously until the end of the play. By that I mean, he had Hamlet observed, not assassinated, in the mid-play. Only at the end of the play did he move to action that way, and when he did, he tried to set it up again so that he survived. He set up someone else (Laertes) to kill Hamlet at the play's end, then tried to stack the deck with poison. Claudius is immoral, but he does act.
By contrast, Hamlet waffles. His father charged him with vengeance, and even though the ghost gave him supernatural knowledge, he has to test it. He has to confirm that his dead father's ghost is telling the truth. He also doesn't kill Claudius because Claudius is praying. He also makes mistakes when he does act. The biggest of these is killing Claudius. Finally, in playing mad he damages his reputation for the future. People might follow a harsh king (Claudius), but who would follow a mad, waffling king?