Earthshock was the last one I saw, it's always been one of my favorites, the fifth Doctor's catch phrase came from this one, the Cybermen stuck halfway though the door was awesome and I LOVED Tegan's "Mouth on legs" line.
Wait, the 5th Doctor had a catchphrase?
I just finished parts 3 & 4 of "Spearhead from Space" last night. Liz Shaw is absolutely adorable!
(It was weird though. Throughout part 4, I kept feeling like I was watching part 5 of 6, not part 4 of 4. I kept waiting for the next cliffhanger and all I got was a resolution.)