In no order:
-The Villa [CV 64/LoD] : A very beautiful stage. Lets hope they put it into more CVs. There's Garden of Madness which is in 2D, but in my opinion doesn't even come close to the Villa.
-Ghostly Theatre [LoI] : Another absolute favorite. Even though LoI is well known for it's copy pasted rooms, the environment for this place is still good. Also love the music and also has my favorite enemy as a boss.
-Inner Quarters [AoS] : Looks a lot like Ghostly Theatre, just in 2D.
-Pinnacle [DoS] : Snowy rooftops and beautiful structures wins this for me.
-Treasure room [CV4]
I seem to have a thing for classy and sophisticated areas.
Oh and welcome to the forums. ^^