I played through all of them on Legendary for the first time, as a PC player who had played every other classic PC FPS (Half Life, Doom, Quake, Wolfenstein, System Shock)

Halo is horribly inferior to all of those, and I'd genuinely say that Halo 1-3 on Legendary are outright bad games. They're incredibly sluggish with terrible encounter balance, Halo 1 reuses all of its levels during the last act, Halo 2 has bullshit throughout the entire campaign, and 3 has terrible levels like Cortana that are a chore to get through due to enemies like Needler flood.

The only reason that Halo ever had any popularity is because it was the biggest fish in its pond (console fps) and it invented mechanics such as 2 weapon limit, regenerating health, and bullet magnetism. The only thing Halo might have that lives up to games like Doom or System Shock 2 is its level design, which is sometimes great like in Silent Cartographer, but the core combat and enemy encounters are hilariously sloggy to get through and inferior.

It isn't Halo's fault that other games copied mechanics like regenerating health, but even in the Bungie series it is used as a crutch for bad design. No other FPS of its era would lazily throw 7+ waves of bullet sponge enemies (without noob combo) like virtually every other Halo game does. I can say this as someone who has played most of the classic FPS games from its era.