Gosh, I just loved "Four To Doomsday"!
I always love super-intelligent aliens or computers that still manage to miss the obvious, The idea that these aliens aren't able to travel through space very fast, and thus their trips to Earth about back are spaced out by centuries upon centuries. Having humans on board who represent us at various points in history is a lot of fun. Plus, it was fairly respectful of the non-white races it presented in the episode, which always helps.
I also enjoyed that this episode had the Doctor floating in space and using a cricket ball to hurl himself in the proper direction. Considering this Doctor wore an over the top cricketer's outfit, they sure didn't use cricket a lot aside from "Black Orchid."
The arc of trying (and failing!) to get Teegan back to her day and time is always nice.
Adric being a boob and falling for Monarch's bullshit is also nice. It makes both Adric AND Monarch believable.
All and all, a pretty smashing episode.
"Cave of Androzani" is some of his best, simply because they finally gave him a personality.
I like viewing the 5th Doctor as a shy, unconfident, more fragile and subdued Doctor. But he was much more fun as an arrogant risky-taker in "Caves".
Here's a youtube I made all about his celery stalk:
http://www.youtube.com/watch?v=CpHy432XIb8 