I’m with @unmanneddrone‘s sentiment on the universe, design and aesthetics. I may be more of a quiet fan of Halo’s universe and story than the actual games themselves, but I enjoyed both Halo 1 & 2 and read the original Fall of Reach book, so I got a pretty good dose of lore.
I personally always love the sci-fi premise of a fledgling deep spacefaring human race that spreads and prospers until it inevitably incurs the wrath of some ancient race, whether by misunderstanding, sheer malevolence, or reasons unknown, that systematically wipes the human race from the universe. I’m a sucker for last stands, long odds (UNSC needed 3:1 odds to even come close to victory), and sacrifice, sometimes for very little gain or just to be able to fight another day and be defiant until the bitter end. Halo has this in spades. See also: Mass Effect (Reapers/First Contact War), Babylon 5 (Battle of the Line), Space: Above & Beyond, Freespace 1 & 2.
You’ve seen it a million times in sci-fi, but I’ve come to love Bungie’s take on the chunky, utilitarian space marine tech and ships (with awesome, pretentious names), their glossy and sleek Covenant armada, the exotic alien locales and Forerunner architecture they’ve crafted, including the Halo(s). As much as Master Chief has become the unintentional mascot of a younger generation of gamers that only play FPSes and “dudebros” who only care about Halo, Call of Duty, and Madden, I still think the Spartan II’s look and are fucking cool.
Also, Marty O’Donnell brings it all together.