Add me to the consensus that Halo 3 had the best Campaign of the series. Cortana is awfulawfulAWFUL, and there aint much of a story, but the enemy encounters and general level design is just fantastic, really shows off the Halo-style of versatile combat, varied AI, and great overall pacing. Really fun, very replayable. I'd put Tsavo Highway, The Storm, The Ark, and the Covenant up with any level in the series as one of the best.
Reach didn't do that for me. A combination of the different weapons I didn't like as much, an even worse story with characters I can't even remember the name of, much like why I should care about any of them, and just a real middling feeling to the whole thing. Halo 3 builds up brilliantly with Sierra 117 and Crow's Nest, easing you into the campaign and the challenges you're gonna face, then it pretty much kicks into overdrive for the next couple levels, dialing down a bit for a a good Flood level that's short and sweet without overstaying its welcome like many Flood levels, then back to the awesome of The Ark and The Covenant. The entire Halo Reach campaign, I was waiting for it to get great. Instead, I was during boring ass spaceship missions and boring ass flying around the city stages that just draaaaaag on forever.
Outside of Cortana, I could replay Halo 3 on Heroic or Legendary with some buddies whenever. I NEVER feel the need to replay Halo Reach ever again.