Seems I'm not alone in finding this quite a difficult one to choose. Led Zeppelin II stands as my second favourite record by the band, for now at least - as a collection of songs it's very strong and very solid and there really aren't many better sets of memorable hard rock riffs out there. When listening to it, I often listen to the full album in one go.
I voted for Side Two in part because it has five songs instead of four

but also because, as much as I like the sonically and lyrically beautiful Thank You, I think the latter side has the superior acoustic track in Ramble On, which not only boasts excellent bass and Tolkien-inspired lyrics but also is a fine example of the famous 'light and shade' approach. I find picking a favourite song from II to be tricky as the overall standard is so consistently high but if I was forced to pick one highlight I'd take Heartbreaker, which has a fantastic riff, great Plant vocals and a superbly sloppy guitar solo.
Having said that though, the track that I've played the most according to my iTunes library is What Is And What Should Never Be, which of course is on Side One. Decisions, decisions...