It normalised the twin twick controls for shooters that are still used now.
It paved the way for big esports tournaments with high cash prizes.
It litetally spawned a flood of copycat games.
It introduced and made vehicles viable in a fps game to the point that the half life then introduced the buggy and unreal tournament 2004 introduced vehicles.
It had big scale battles that allowed the player freedom to choose how to approach these battles in various tactical decisions. Prior most fps were small set pieces or corridors.
But because Its an MS 1st party title and xbox is shit, it dont get the respect it once garnered.
Metroid prime is fantastic. I loved that game. But in terms of impact. Its a no brainer.
Twin sticks, sure. It's definitely the best feeling console FPS there. It introduced the "weighty" controls that a lot of console FPS have adopted.
eSports? Maybe console specific eSports. Halo wasn't close to things like Counter Strike though and was never really taken seriously.
Which games would you consider to be Halo copycats? Honestly can't think of any prominent/popular ones. Destiny is the closest thing for obvious reasons.
Vehicles, sure I suppose. I'd say BF was the influencer there but Halo definitely was one of the first regardless.