Halo by far.
I never had much dificulty with Gears on insane, sure it was challenging but still doable in a normal way.
Halo (specialy Halo 2) was hard as nails on legendary, people already mentioned Jackal snipers that could oneshot you (actualy they could oneshot you on heroic I recall), but elites had a lower TTK than you, in particulair the jetpack Elites in H2, making them tricky to kill.
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Have you tried Gears 4 on insane? It is much more difficult then all other games.
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I have not, but I have played 1 & 2 on insane way back when they were new. But I can see how on Gears 4 that difficulty could make it harder, but that is mainly because of those kidnapping assholes getting gameovers when you let them escape.
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Gears 1,2,3 and Judgment play more or less the same on insane. Gears 4 is much, much harder. Enemies are stupid smart. You can't win a chainsaw fight. Damage is insane against you. Enemies have a shit ton of health.