: Mine was Part 2 of The Origin of Species ("Lava Me Tender"), which
: I impulsively chose because it was one of the smallest downloads on the
: list. It was so meticulously designed, in both gameplay and aesthetic
: terms, that the next few maps I tried came off as unforgivably sloppy
: let-downs.
I still feel vaguely ungrateful about Origin of Species not having been completed, what was done having been so good, but that's not the topic.
The first third-party maps I played were Marathon 1 net levels, and I had a little utility that promised to rename everything for you before you launched the application with its hard-coded assumptions of where it was getting its data. I don't quite remember what they were, though. I also have memories of some small third-party scenarios, including one that had you stopping a further Pfhor invasion of the Tau Ceti colony with Leela's help (made before Marathon 2, obviously!), but the first scenario that really stuck in my mind was the non-linear Operation Tantalus.