I have absolutely no idea who said that to you about the story. It's good for a turn-of-the-millenium FPS, but it's not amazing or anything. The game writing peaks with Halo 2 in complexity and ambition, with later games being good at hitting vibes or emotional cues, but lacking the intrigue and twists that 2 had. I remember the expanded universe being interesting, but Halo's continuity eventually started being dependent on reading all the books to understand the games after 343 took over from Bungie and the whole thing really suffered for it.

The one thing I was impressed by in Halo was enemy AI. That was really well done. Somehow, despite it being so old, most games don’t bother making AI like that today.

It really is one of the best things about the games. Each species of enemy has its own AI package with distinct behaviors, and depending on the game, there are up to three, I think, variations on that that scale with rank. Higher ranking Grunts/Unggoy are less likely to panic if their leader is killed in most of the series, for instance. It's really weird that this kind of thing doesn't seem to be common outside of high fantasy games. Zelda does it, but Ghost of Tsushima doesn't have much difference between low level bandits and high level Mongols other than their health bar and damage output. I might have to check out the handful of fantasy FPS games to see if they have similar enemy variety.