I would tell you to start with the season that started with the current Doctor. Then go back and start on a season when a new Doctor starts. Every doctor has his own arc and don't really cross over each other much. In some of the newer ones, you won't make the connection to some of the "older" characters that make guest spots, but then when you work your way back to the season where they show up originally, you'll be like, "Oh, OK I get it now". Someone else put it in another of the comments that the great thing about a time traveling show is that you don't have to watch them sequentially. That's kinda true, but I'd say its nicer to watch the episodes of a season that way, and to watch a Doctor develop from the start of his first season as the Doctor. Good luck with catching up.