The ones I'd recommend for level making and playing:
Knytt Stories is nice. I have a few well-received levels for it, and I've helped push a collab level up to second place in a high-stakes level designing contest. There is a mod for it that adds a lot more content and functionality, but nobody really cares whether your levels are made vanilla or with the mod in mind
PuzzleScript is a special case, because by its very nature, all games designed in it are open-source (unless you're using a hack) and can be very easily tweaked, such as adding/replacing levels.
Celeste has a fan-made editor that's still in the making, but is still fairly capable and easy to figure out. The only scripting involved is minimal (so far) and should be easy enough to figure out if you poke around the Discord server and ask the right questions.
The ones I wouldn't:
Knytt Underground is well-equipped for hard platforming, but it's designed differently than Knytt Stories such that you have to put a lot of decorative props and effects everywhere if you want your level to be aesthetically pleasing, and figure out an undocumented Python-based scripting system in order to include teleports or items. I haven't made anything with it yet, for these reasons. No-one else really cares about making custom levels for it, since
the only working editor was near-impossible to find for a while.
Portal 2 has Hammer and the in-game editor; hard to use but very capable and easy to use but very limited, respectively. Unless you're fluent in Hammer, however, I wouldn't recommend getting it just for the custom level capabilites. There's a lot of elitism by people who create maps in Hammer, not helped by the fact that a majority of the public level submissions made with PeTI are horrible.
VVVVVV similarly has an elitism issue: your level is seen as shovelware if it doesn't have advanced scripting and manually placed tiles. Both the built-in editor and the fan-made editor are equally capable, though.
Super Meat Boy has a large collection of custom levels, but the editor isn't very intuitive, and doesn't even include aligning objects to a grid (which makes me question how the >280 main levels were built).
