All Guild Wars chapters are stand alone games, so no matter which one you get, you will be able to play it without getting the others. If you decide to get one now and another later, you can link them together, giving all your characters access to both games.

In order, the chapters are Prophecies, Factions and Nightfall. Nightfall is the best of the lot imho, so if you only get one, I would suggest you to get that.