I'm doing a complete rewatch at the moment, in preparation for the special next month.
It is a brilliant show, with excellent casting people!
Edit: I dont think any of the below count as spoilers since it's widely known the companions change regularly and the doctor changes - there's no details how how things go down
Once Tennant takes over in season 2, the show really hits it's stride and starts to get better and better
David and Billie are just fantastic together, the next companion, Martha, I didn't enjoy my first time round, but she gets pretty cool in the season 3 finale, and I liked her much better during my rewatch (I think the first time I just wanted Rose back).
Then Donna. Oh Donna, she is just phenomenal, and season 3 has one of the most brilliant finales, then we get some specials that are not as good, till Tennants last episodes and they are amazing!
The season 2 Finale, season 4 finale, and Tennats last episodes should be used as a soul-detector test, if you aren't balling your eyes out, you need to consult your doctor as you may not be human/have a soul.
Matts run with Any and Rory is brilliant, but I think it has slacked a little in the most recent series, but it does also feel a bit like it's all been building towards the special next month, so it might all pay off.