There are a lot of good choices here, and probably more among the Classic Who episodes I haven't seen -- PBS was showing Four's run when I started watching, so I'm still behind on my One, Two, and Three viewing, and have missed some of Seven's episodes as well. (Which is what I get for leaving my home state to go to college.)
But I have to go with one of the obvious choices here, which also happens to be one of (if not the) first Who serials I ever saw: "City of Death."

Four and Romana II being cute running around Paris; a cracking Douglas Adams script; and to this day, the Jagaroth mask still creeps me out. Plus there is bonus John Cleese critiquing the TARDIS as if it were a work of modern art! Seriously, if you don't love this episode, I think you may be lacking a tiny piece of your soul.
( the rest of the days )
But I have to go with one of the obvious choices here, which also happens to be one of (if not the) first Who serials I ever saw: "City of Death."
Four and Romana II being cute running around Paris; a cracking Douglas Adams script; and to this day, the Jagaroth mask still creeps me out. Plus there is bonus John Cleese critiquing the TARDIS as if it were a work of modern art! Seriously, if you don't love this episode, I think you may be lacking a tiny piece of your soul.
( the rest of the days )