If you've played the series at all, you can make solid arguments for 1, 2, 3, and Reach all being the best game in the series.
But Halo 2, campaign wise, was the best of them all. With deep exploration into the lore, it paints the enemy as not just something to shoot- but something to relate to. It was the first game in the series to also paint it as a cinematic experience, the whole game felt like something to transcend gaming, and it did a wonderful job at it.
A hierarchy about to crumble due to lies, humanity trying to not go extinct, and a parasitic species that interrupts, trying to tear down anything living at the seams- this game was legendary. Though my opinion is with nostalgia goggles, it still stands the test of time. Especially the remaster.
I won't even mention the soundtrack or the excellent one liners.
If you haven't played, I think you should give it one solid play through.