Which one and why? Personally Halo CE is just brutal for me.
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Solo LASO without using a guide? [spoiler]Tried it in Reach and got near the end of winter contingency, but I was killed by the damn dropship's AOE right before leaving on the falcon. That's when I knew LASO was not for me.[/spoiler] But if we're talking about legendary, Halo 2 by far. I might just be a n00b, but it took me around 10 hours to beat the whole thing by myself. Also note that I had only played through it once on heroic prior to trying legendary.