The nihilist thing has gone past. Starlin doesn’t even go that direction anymore. Starlin doesn’t write an evil Thanos, but recent writers are.
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Darkseid. It's a close one, but Darkseid is more evil.
Thanos is a tormented character whose motivations have more to do with his hatred and resentment at life than his actual pleasure in causing the pain and suffering of others. In fact, pain and suffering are the phenomena that cause Thanos to hate Life with such Passion. He perceives Life, the Physical Universe, to be an actual Hell, thus he justifies his Nihilistic Vision with such disgust for being alive.
Darkseid, on the other hand, has no other motivation for death and destruction other than his own egomania and complete disregard for all life and all creation. Darkseid has no personal attachment to, resentment of, or hatred for the Universe - he simply desires to control it and will torture or kill any and all who oppose, disobey, or simply annoy him.
Although Thanos has committed some of the most evil and unforgivable acts imaginable, he has experienced empathy throughout his journey, even though he returned to his true nature as a villain. Darkseid has never had any such enlightenment nor has he any interest in it. He's just evil and he's happy that way.
Conclusion: Darkseid is more evil than Thanos.
