I have been watching the fourth season (thirtieth for us die hard fans) on Sci Fi and catching up on the second season on PBS. This is one of my favorite shows of all time and I always liked Nyssa of the older series.
Donna, Martha and Rose are incredible companions. It is hard for me to choose, but I'd go for Donna with Martha being the runner-up. I still love Rose before anyone gets up in arms.
Does anyone have a favorite companion?

While we're at it, who is your favorite Doctor? David Tennant is my favorite, with Tom Baker being second. All of them brought in something special, except for Paul McGann who did not have time to really develop his Doctor.