I loved one flag in H3 Last Resort; many fun nights were had playing customs 2 v 2 on the same console. Fat Kid on that forged map in foundry would have to come a close second though ... I can remember frantically tossing grenades to the ground trying to get a needler on the next spawn before fatty got too close...
Halo 4 doesn't seem to have the same depth of custom games as H3; whether it's because of the nerfed forge or lower population I can't tell...
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Any kind of infection customs in Halo 3 was pretty fun. Can't remember the name of it, but the one where everyone has active camo, snipers only was pretty intense.