The Doctor travels with human (most of the time) companions, and we get to see him in different times, on different planets, and see different baddies. Some of them are from the classic show, and some are brand new. It doesn't matter though, because they're all awesome. In fact, some are downright scary. My favorite episode is titled Blink, and guest stars a pre-fame Carey Mulligan.
Who's who (weirdly, this is now a pun): The doctor can regenerate, making it perfect when an actor is ready to move on to different projects. I'll talk about my favorite, and the first Doctor that I watched (people tend to be attached to the first actor they see in the role). You may remember him from Harry Potter and the Goblet of Fire as the super creepy Barty Crouch Jr., or you might have seen him in show Blackpool. He has fabulous hair, and I found his seasons to be the most emotional. Granted, I cry at just about everything TV/movie related, so it's no surprise that I cried like every episode during his run.