I'm currently subscribed as a VIP player but I'm already planning to budget my Turbine Points for when I decided to cancel and just go back to Premium. (I'm the type to probably continue subscribing but it's a backup plan, just in case.) Anyway, my favorite so far is Evendim (levels 28-40) because of the intricate architecture of setting and just the plain beauty of Lake Evendim. There are also tons of quests and deeds here so it's perfect for your mid-level characters. I'll probably go for Mines of Moria (expansion pack) next so I'll have access to the Runekeeper and Warden classes as well as Moria itself. What's your favorite quest pack so far, and why would you recommend it?
I really like Evendim. I have always loved the region since it came out. It is beautiful and the story is really great if you follow the whole thing. Since they redid the region, the quests are even quicker and you will level up very fast if you go there around level 30. I would recommend it.
For solo content my favorite region is Evendim. It's such a beautiful place and most of the quests are do-able alone. However, other than annuminas, there isn't a whole lot of group content in the area. For more group based content, I love Angmar, especially going through the carn dum area. Angmar is just so big and there's plenty of group content to do to get your character up to and even a little past level 50. The Rift Raid also comes with Angmar and it's my favorite raid in the game.