Halfway through Halo 3's campaign on Legendary. Now I remember why it's easily the best Halo game. Campaign is way better paced, and balanced than any of the games that came before or after it. It took the open ended level design of Halo CE, trimmed the fat, eliminated the vast majority of the repetitiveness and asset recycling that Halo 1 and 2 were infamous for, made the brutes a much more enjoyable foe to fight, and less frustrating than in Halo 2. And most importantly, it never seems to starve the player of ammo, or the optimal weapon to use for each scenario. A bit of scavenging and you'll always find the right piece of kit for the job. The deployables add a ton more fun to the combat too. Especially the bubble sheild, deplyable cover, and the anti-grav kit that allows for some interesting shortcuts to certain vantage points and opens up your attack options. The brute vehicles and weapons are a riot to use as well.
Only level I didn't enjoy was "Cortana" a minor blip on what's essentially a fabulous campaign.