I'd say having a big ego has to be considered relative their powersets and accomplishments. If so Doom is actually lowest on this list because if you actually look at what he's accomplished his boasting and pride really seem rather insignificant by comparison considering he can back it up most of the time. Darkseid ranks relatively highly because for a being with an ability to erase people from existence in an unstoppable beam that can track through space time he's pretty pitiful.
Thanos generally doesn't actually boast too much and tends to stick to facts as far as he is aware of them... He does challenge those above his station/powerset but he generally has an ace to back it up when he does or is in a position where backing down isn't really an option
Doom definitely has the biggest ego, however if i had Darkseid's power I would have the biggest ego.
doom
darkseid
lex
thanos
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DS. He forces an entire planet to worship him. He wants his subjects to think he is almighty. Heck even when he tripped down the stairs, the characters around him were afraid for their lives for having witnessed it.
doom and thanos have similar feats (daring to challenge space gods and in some instances, murdering/usurping them for personal gain), but thanos had his soul peered into by adam warlock and found a form of self-loathing deep within him. (minus points), doom iirc has no inherent fault, or at least wouldn't even acknowledge it even after warlock told him he saw it with his own eyes (thanos kinda acted like a little bytch tbh)
lex has his moments, but his ego can be confused with an obsession to get one over on superman.
Doom is just a reaaally smart man with some magicians tricks who talks a whole lot of self-aggrandizing and shit for someone who refuses to let the world look on his face.
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