Which one is your favorite?
Sorry if I missed any, I have no prior knowledge of anything but the games (no idea what characters are in the books).
*Reason 343 Guilty Spark was not listed- I didn't exactly consider him a "villain", more of a machine programmed to do what it's supposed to, save the galaxy by destroying the Flood, be that he may have to activate the rings.*
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I found the Gravemind's little moments highly annoying not to mention that boss fight tease + descent into Floodland in H3 very pathetic. I found the Prophets to be weak and pathetic. And I found Tartarus to be unreasonable. So that just leaves Didact and Thirsty Grunt... I vote for Thirsty Grunt!!