I think Sudan, Egypt, and Libya would be contenders. I suppose there's probably a general principle at work that if your country's borders were decided not by geographical or cultural boundaries but rather by an occupying bureaucracy, then there's a high probability of squareness.

Or if your territory is made up partially of land the ownership of which isn't a pressing concern for anyone because it doesn't support a population and has few resources.

The western part of the boundary between U.S. and Canada is unusually straight, but I suppose that was an expedient agreement at a time when neither side wanted to go to war over it. Not that relations were totally peaceful or friendly.