Mon-El is one of my favorite Legionnaires. He might even be my number 1 favorite. In general, I love the strong guys on a team from Superman to Thor and everyone in between.
Mon-El was mistaken for Superboy's older brother in his first appearance, but that's kind of what he was. An older, cooler version of Superboy. He was a good guy, but the writers were allowed to let Mon-El cut loose and have more fun with him than they ever could with Superboy/Superman.
Here's Mon-El going straight-up gangsta on Superboy when Kal is behaving like a spoiled brat because Wildfire was elected Legion leader. It's from Paul Levitz's very first Legion story -- Superboy 225.
It was also the first issue not drawn by Mike Grell since Grell started on Superboy 203. The above was drawn by James Sherman and inked by Bob Wiacek.
Going back to our discussion of Legion artists,
James Sherman was the best of the post-Grell pencilers until Keith Giffen took over years later, but unfortunately Sherman couldn't handle the comic monthly once it went to giant size with 231, so his art appeared more infrequently. Another unfortunate thing is that they often gave him crappy inkers like Jack Abel who overwhelmed Sherman's pencils making them unrecognizable. When they gave Sherman a great inker like Bob McLeod, Sherman's art really shined like in the first 2 chapters of the Earthwar Saga (the intro splash page of which is posted on page 2 of this thread).
Here's another great Mon-El moment. It's from Legion of Super-Heroes v3 38. Post-Crisis, Superboy is revealed to be a creation of Legion enemy the Time Trapper. Superboy sacrifices his life to save the Legion and dies in Mon-El's arms.
Finally, perhaps Mon-El's most triumphant moment. It's from Legion of Super-Heroes v4 4. Mon-El has had enough of the Time Trapper and utterly destoys him. However, it's a Pyrrhic victory as this sets into motion a chain of events that undoes all of Time Trapper's machinations to the timestream, creates an alternate timeline where Trapper's old sidekick Glorith is returned to life, and allows Glorith to recreate the Trapper's original timestream, but insert herself in his place thereby becoming the new Time Trapper and a great Legion villain for years to come. As much as I didn't like the 5-Years Later direction, this development was its crowning achievement.