Luthor is one of DC’s most inconsistently characterized villains. At his worst, he’s capable of a far higher bodycount than the Joker - in the way that an Atom Bomb has killed more people than Ted Bundy. Is the A-Bomb more evil? Kind of hard to say, isn’t it? Ted Bundy’s crimes are certainly a vastly more visceral kind of evil.
But at the same time, Luthor is often characterized as an “anti-villain,” or even closer to an “anti-hero” at times! There’s times when his criminal actions have well-meaning motivations behind them, times when they’re motivated by petty jealousy but rarely cause much lasting harm, even multiple times when he allows himself to be captured on purpose in order to help someone, like the people of Lexor, or Cal Anderson.
And yet there’s also times when he’d blow up the whole world, murdering everyone in it, just to make Superman sad. Once, Luthor cured a debilitating disease, only to inflict it again immediately, out of spite. I mean this guy is not really one single consistent character! Like any character, Luthor is a series of ideas and concepts that could probably be made into multiple, distinct, utterly irreconcilable characters.
By contrast, not every Joker is the same, but at best, he commits crimes because it’s fun and doesn’t really care if anyone gets hurt. At worst, well, we all know how bad he can get.
So I’m giving the award to Joker, because he’s more consistent, and because my favorite versions of Luthor are all… not really evil at all, in the final analysis. I know there’s versions of Luthor who are evil, but frankly, I just don’t find them that interesting in comparison to a more sympathetic, even quasi-heroic Luthor.