Will always stand and say this game has the most cleverly designed Campaign in the series, given how interactable the environments or levels are. Their so much more complex than just 'verses'. Take the chase sequence from Phantom or Shadow in the hallways, the atmosphere links itself to the environment and gives you interesting interactions. Do you want to be rewarded, and kill the enemy there, or run away and not risk it? Or simple things, like ricocheting attacks from the dragon statue's beam. Alot of these gimmicks give this game an interesting legacy that is very much missing from the others.
I feel it has the perfect balance with its enemy/combat design by giving you freedom on how to play, but pushing you with the complex enemy behaviors too. The game's combat is more simplified compared to other later entries, yes, but each attack feels like it has more meaning, and weight to them. Take Roundtrip for example, in all the other entries you wouldn't really use it because its more for flashiness, here due to the more weight each and every attack has due to a more limited moveset, it feels more necessary. Alastor/Vortex itself as a technique gives you tons of mobility be it by air, or the ground and the guns have really interesting knockback quirks that it's enough to stand on its own. Other games in the series really aren't the same as it and I really appreciate it for that.