: Personally, I'd lump it into the science fiction category with a bit of
: fantastical stuff added in.

As an SF/F writer I see a lot of this debate, and I don't think it's useful to treat the two as binary categories, and maybe not even as two opposed poles. Sometimes you'll get a story (Silently and Very Fast) that's pure SF but reads like fantasy. Sometimes you'll get a story that's very science fictional and very fantastic.

Halo is certainly space opera. It's got a ton of fantasy in it, but that fantasy is conveyed through science fictional devices and ideas. It's not very hard (this isn't the same as 'good') and, tonally, it's most happy when it plays in the epic fantasy sandbox - a story of heroes, villains, clashing civilizations. Occasionally, though, it dips its toes into science fiction. Halo 4's intro briefly asked some interesting questions about dehumanization and treated the Spartan program as more than a factory for candy-colored supersoldiers. Bungie liked to cram its backstory, stuff like the Terminals, with nods to harder SF.

Marathon was great science fiction and I won't hear otherwise!

In general I think that Halo, like most space operas, is telling an archetypical story using science fiction props. This is a totally fine thing to do, and I don't condemn it.

Definition arguments are really tricky and often don't go anywhere interesting.