Always have a soft spot for Genesis. I remember watching the repeat showing in the 1980s and being stunned when my Dad told me that "Ronson" was his art teacher at high school.
Spearhead from Space was one of the first stories I bought on VHS and I watched it so many times, so that's a definite favourite too. Oh, and Ark in Space.
Glad to see another who likes Ghostlight! Most everybody else I've met thinks it's though.
Your first is always your favorite, so mine is The Robots of Death. Runners up: Nightmare of Eden, State of Decay, The War Games, Mawdryn Undead, Dragonfire
And on third, beacuse I gotta have a Pertwee episode in my list: Inferno - Over the top acting, weird scifi stuff, escalations on top of escalations, and the Tardis console flying free of the Tardis.
Pyramids of Mars. The City of Death. The Key to Time. Trial of a Timelord. Anytime Jon Pertwee, Roger Delgado and Katy Manning were on screen it was magical.
I'm with you with Genesis, apart from that I love The War Games and The Caves of Androzani.. Additionally, I'm quite fond of The Time Meddler, but that is maybe due to the fact that I watched it just recently :-)