Earthshock, Arc of Infinity and Snakedance (Fifth Doctor).
Three excellent serials. Adric was too much the whiner in Earthshock but the writer made good use of his character and gave him an interesting death.
I was glad to see Omega again in Arc of Infinity but Stephen Thorne's performance in The Three Doctors was better that Ian Collier's.
The Mara are an interesting villian that I wouldn't mind seeing again on modern Who. That said, while they get some good development in Snakedance, Kinda was a better story.