While the 2kxx series of episodes are pretty good (undeniably), I wouldn't recommmend it if you're getting back into SMBX. For one, it has a
ton of inside jokes and is based, in part, on the history of the SMBX Community, and, two, they are usually hard. Like,
really hard.
I would recommend the episodes from
THE SMBX CLASSICS COLLECTION by Knux (The collection, not the episodes themselves). Of those, I would probably start with
Valtteri Island. It's good and short. Then
Apocalypse of Foroze. That episode is
insanely good (It spent 3 years in development), and it'll give you plenty of play time, as the episode has so
much content. The
Great Empire Trilogy is pretty good, too. But it doesn't outshine Apocalypse of Foroze any (Not by a long shot).