The other Brainiac 5
It's hard to pin down a defining moment out of a relatively small pool of, well, moments, good or bad. The Legion cartoon and its comics were both cancelled after just two years, after all.
Still... there are some things classically Brainiac 5.
One: Having a cunning plan to turn the tables on space pirates that steal from your friends? Good!
Having installed a backdoor program to grant you personal control over your fellow Legionnaires: Bad.
Everyone finding out like that? Really bad.
But that's Brainy: If a little SCIENCE can save the day and keep his friends safe, well, damn the consequences, that science is getting used. It's, uh, far from perfect--the comics and second season were both a little uneven-- but it's some kind of moment.