I would say that CE, as it's the only one I actually finished and had the most interest in. It was a huge deal back then and while all other games could be argued for (from the original trilogy), it did feel like the most self-sure entry and one of the first console FPS games that are still fully playable to this day.
I am not a fan of the series, but when it comes to the soundtrack I still listen to it quite often. Also, It might not be my cup of tea, but it's hard to argue against its cultural importance.
Vote the Mayor for Mayor!







