I was very fortunate in the 90s to receive a lot of sets from the desert subtheme. My Temple of Anubis (Pharaoh's Forbidden Ruins) got explored plenty of times. And boy, were those good times. A few years later, my parents bought me a couple of Dino Island sets. I don't have any fond memories of those. Recently I started buying Jungle and Orient Expedition sets to get a taste of them. Now I love all subthemes almost equally except for Dino Island. Amazing prints and rare parts can be found here.
Picking a favorite is hard. But I went with Amazon. Purely because it deserves more love on this poll and shouldn't be this far behind. There are some amazing sets here with a unique classic style. I guess if I were to pick a favorite based on quality and variation, I'd have to go with Orient Expedition. Lego truly delivered in this theme.
If there was to be a fifth subtheme, I'd say this:
Right now, we have the City Jungle sets, which does have a jungle setting with ruins. Sadly, there's a clear emphasis on modern vehicles. The ruins are only a small part. So it's still untapped potential in my opinion. Another idea is to have an Islanders subtheme. With an ancient tribe living in a dangerous environment, filled with temples and broken pirate ships. A lot of this has been covered in other themes.
Luckily, we have the power of modifying and we can put Johnny Thunder wherever we want :-)