Honesty is the most important quality, and I mean that sincerely. It doesn't mean that the politician is like the comedic character who cannot tell a lie, and blurts out that people look awful in those clothes or whatever else he's thinking. It does mean that if he develops a reputation for deceiving other political leaders, his power will diminish considerably.
Intelligence is of course important, but the ability to build a team and coordinate their work is more important than rocket scientist brains.
Personality is crucial, because at any moderately high level of government, a successful politician is one of those people who simply "takes over the room" when they come in, effortlessly. People want to hear what they have to say, they are pleasant to be around, and if they ask you to do something, you do it.
