There is no bad place to start, since they are just collections of essays, so you don't need context or anything. I started with Assassination Vacation, however, and I consider that her best work.

I would start with "The Partly Cloudy Patriot" mostly because I like the title and what it says about the author. To me it suggests that seriousness and integrity are the underpinnings of her humorous musings on politics and US history.