I think the only one I've actually beaten is "Descent v1.3". Personally, though, I wouldn't even call it a scenario; it's just a one-level map file full of enemies, without even a story. I also think I played it on Kindergarten, in order to fist the Juggernaut "endboss".
That said, the subject could be read in two different ways - "I have beat this scenario" or "This scenario has me beat". But I think the second list would also be empty for me - with most scenarios it's just that I never bothered to finish them, not that I got stuck or anything.
: One of the things I have begun to love doing in my spare time is challenging
: myself to beat custom-made M2 scenarios I find off of The Archives. So far
: I've beaten a few of them, and I'm still going on a rampage in beating all
: the M2 scenarios I can. Which scenarios have YOU beaten?
: So far I've completed: 9 Minutes 37 Seconds
: Pure of Heart, Brown of Trousers
: What Ho! Jeeves
: Columned Fun!!
: Outerspace
: I'm working on beating Empire of the Serpent Sun.