Stranded in Space has the best intro, ending and end credits sequence of any MST3k I've seen. The riffing and host segments are kind of off but I love "ooh hoh that's some good smack!"
Monster A-Go-Go is another great one, if only for the last 20 minutes. It's the kind of episode that's the most fun to talk about with your friends and tell other people about but that's not that fun to watch.
Pod People has been a total cunt to find on the internet so I still haven't seen it.