"Ghostlight" and "Curse of Fendric" are far superior, and in a lot of ways those two episodes were some of the best of Doctor Who (yes, I said it, I'll say it again if I have to!) but "Survival had its moments.
The best thing about Sylvester McCoy, other than himself, was having Ace as a companion. Finally we had a companion who had an interesting story. Ace was a strong yet damaged girl, and very much adopted by the Doctor. Survival in many ways is her story, the otherworldly events a mirror for Ace and the other teenagers from her neighborhood, the kids society abandons and who become animals on the street.
The very last moment of "Survival" is amazingly beautiful. Ace sitting on the ground, thinking she's lost the Doctor, and holding his hat and umbrella. With a tear, she firmly puts the hat on her head, as though she is going to take on the Doctor's role, and as she does this we the audience see the form of the Doctor step behind her, offering a joyfully confident comment to Ace. "Mine, I believe."
McCoy's last words are often quoted, and it is a fun little speech he gives about all the wonders of the galaxy, but I think it is that prior moment with Ace that is truly memorable.
McCoy and Ace are my absolute favorites. Their relationship was complex and engaging. If only he'd not had that damn question-mark jumper!