IMO, King Sombra is easily the baddest of the bunch. He was a cruel tyrant who delighted in making his own subjects suffer. Queen Chrysalis, on the other hand, does at least care about feeding and looking after her changelings, even if she's willing to threaten the ponies for it. Trixie was boastful and stubborn, but this was more out of inner insecurities than true nastiness. It took a magic amulet to make her a real threat, and after being separated from it, she felt sorry and willing to reform (though I'm sure she still boasts sometimes). Discord is mainly just chaotic and mischievous - although turning the Mane Six against each other was pretty devious - and finding a proper friend in Fluttershy helped him to reform too (though he doubtlessly still plays non-dangerous tricks for amusement). And Celestia seemed to know that even when Luna became Nightmare Moon, she didn't become pure evil - she was just really jealous and misguided, and rediscovering love and friendship got her back on the right path in the end.