I'd recommend these episodes when they are finished:
Airship Attack 2- the stars concept has been overdone, i know, but the levels are great so far (i am in world 3)
Mario's Kingdom Adventure- I've beaten the demo 100%, and except for 2-2 and 3-3, all the levels are great
I don't recommend these episodes:
Curse of the Seven Stars- too many enemies everywhere, especially in later levels
Clash of Generations- horrific graphics, loads and loads of enemies
Bowser's Dark Plan- too many enemies
I won't recommend these episodes when they are finished:
Toad's SMB Adventure- cannot pass the first level
I would recommend these episodes:
Mix World 2- each level was different in some way, WerewolfGD impressed me with his skillful use of events. The GFX can be meh sometimes tho
The Restoration- good for a fair, fun challenge (I never found the high difficulty UNFAIR...well...once I got good at SMBX that is.