The Legionnaires: Gates
How have I not done Gates yet?
Gates, aka Ti'julk M'rasz of Vyrga. Created by Tom McCraw, Lee Moder, and Mark Waid.
Gates was one of several new Legion candidates introduced early in the reboot. He was (is!) a giant bug with claws and a hood, who also happens to have the power of teleportation. You could if you liked consider him to be an effort to get Nightcrawler into the Legion long after Dave Cockrum had left the book.
Once Gates joined the Legion, he was basically on the team right through the end of the reboot, and after Final Crisis: Legion of 3 Worlds, he shifted membership to the retroboot Legion, where the writers seemed not to know what to do with him. Then he was trapped in the 21st century as part of the Legion Lost title, about which the less said the better.
The great thing about Gates was his politics. He wasn't serving in the Legion voluntarily; he had been drafted by his planet, and was very skeptical about the Legion's role. Plus he either was an extreme leftist or he thought he was. Plus he's a snarky son of a gun.
Another important point: he's one of the very few nonhumanoid Legionnaires in their history. I'd like to see a lot more such characters.
I miss that Gates. If there's ever a Legion comic again, and if they bring back Gates in it, I hope they use that side of his personality. It's useful as well as fun, and I was disappointed when the retroboot writers didn't use it. Here's his first appearance.