I'd like to nominate both Elec armor and Elec melee. About a year before CoH closed down I made an Elec/Elec scrapper, and he ended up becoming one of my most fun and powerful characters.
Elec melee was underrated because most people thought it had poor single target damage. This was definitely not true. It didn't have any big hitters like most sets did, but it had a very solid and fast attack chain. Overall I don't think it was any slower - it would just take like 6 fast attacks to kill someone instead of 3 slow attacks. And then it had Lighting Rod, perhaps the most fun power in the entire game. Seriously, every time I used LR, I smiled. It was incredible.
Elec armor was underrated too, but I'm not sure why. I very rarely saw anybody playing it, but it was very survivable and offered literally unlimited endurance. My elec/elec scrapper was probably my only toon where endurance basically didn't exist, like it wasn't even part of the game. No matter what, I knew that I'd never run out, and after playing so many scrappers with insane endurance issues, this was definitely a rewarding experience.