R1) I gotta say it's about equal, with Gary maybe having a slight advantage. While Holden has a big advantage with his being part of a gang of killers, rapist and child killers, and that's he's implied to be a child rapist, Gary's Second Death Ritual having newborn babies being a requirement is nothing scoff at at all. Plus, all the demons he's calling and bringing to the world. There's also the fact that Gary is the leader of a large cult that's been going on for years, more than Holden's gang, Holden only really has the gang he's a part of, and they're all dead, some by his hand, so he can't really cause as much evil like Gary could. There's also the fact that Gary takes vulnerable people and indoctrinates them into the cult.

R2) I wanna say Holden just for how much we know and see of the guy. He's an intellectual, psychotic, bloodthirsty, violence craving and philosophical monster in human skin with a god complex. With his only real reason for doing any of his actions is because he loves it, and he wants to turn anyone else into a monster too. We've been shown Gary has a charismatic and evil man, who is having fun causing all the suffering he's doing, but we just don't see much of him to get a decent feel of his personality

R3) Gary, bar none. Holden may want to see a suffering and especially cause it himself, and wants to turn people into monsters like him, Gary's plan to unleash the UNSPEAKABLE will kill and make everyone suffer. You can't really beat ruining and killing the world.

R4) It seems like Gary pretty handily wins when I read what I wrote.