I have Mantis at R4. She is one of my go to counter for annoying defenders like Photon. Love playing her, her damage on heavy into sp2 after 20 charges is insane.
Phoenix!
Phoenix Moondragon
BWDO
Lady Deathstrike
Red Skull
All the champs I see mentioned here are perfectly rated except for Mantis and maybe Odin. 90% of the champs people mentioned here will barely get them any wins in Quantum and above, but it's not because of their kits, it's the game the devs have created with all the busted defenders they decided to release right after Serpent.
Honorable mention to this guy for mentioning most of them, Sam Maw Spunk completely irrelevant unless there's a meta supercharging them. Joe Mr. Knight Franken are perfectly rated in B-Cish tier because they're somewhat meta reliant. Cassandra and Havok are already A tier attackers and defenders respectively not sure where he got the idea that they're slept on. He did however at least mention Odin who is a B-Cish tier attacker who is very slept on mostly because people don't realize he can do crazy stuff like Bullseye and Spider Woman, so at least he gets one point for that one lol.
Maybe, “underused” is the better way to describe Cassandra and Havok.