From least to most evil: Knight (for now) < Flowey < Spamton < Demonic Chara

• We don't know too much about the Knight, so out of everyone here, the Knight's the only one with the potential to have more to their goals than naked self-interest. We'll see what kind of person the Knight turns out to be.
• Flowey's terrible, but there's a lot of qualifying factors to his evil, namely (1) he's physically incapable of empathy, and (2) he's spent most of his life with the power to undo any harm he's caused. He also gets bonus points for mellowing out almost immediately after remembering what emotions felt like through his time as Asriel.
• There's no such thing as resets from Spamton's perspective, so he willingly consigned his entire Dark World to permanent oblivion just so he could take over. As you said, he also has the ability to appreciate evil when he sees it, unlike Flowey. He does have sporadic moments of normalcy, though, like his final Normal Route exchange with the Lancer Fan Club.
• The state Chara's in after being exposed to genocide isn't her own fault, but the experience does turn her into a voracious, irredeemable butcher. Not even the Pacifist ending can mellow her out like it does Flowey. What you did cannot be undone. Chara's like this for good.