Arguably, it is the worst. It depends on what matters to you.
I would rank Halo 5 as my least favourite, because the story of a Campaign matters most to me. But it terms of diversity of environments and combat arenas, Infinite is definitely the poorest. That isn’t to say that I think Infinite has a good story, but only that I think 5’s is worse.
I don’t enjoy the open-world. I knew I wouldn’t before the game launched, and my mind hasn’t changed. But even with an open-world, there could have been diversity in the environments. Make an arid low-lying area, or a very mountainous snowy area.
The Endless are…pointless. I really couldn’t care less about them. Firstly, their name sucks. It’s not an intimidating name, and it sounds like they’re trying to impress you. The Flood or the Reapers (Mass Effect) sound menacing without being over the top. They also didn’t name themselves (at least the Flood didn’t name themselves in the Campaigns - not sure about lore overall), and don’t care what they’re called, because they’ll be your destruction either way.
Secondly, worse than the Flood? How? You can’t just drop that on us, and then never even allude as to how for the entire Campaign (end credit scenes don’t count, and you shouldn’t have to complete the Campaign on Legendary).
Thirdly, why do we have yet another faction? The Banished were nothing: just some old Brute. Nonsensically powerful, but also far less threating than they were in Halo Wars 2. Prometheans have disappeared, and the Created were swept under the rug. There was ample factions already - not even to mention that the Didact could have come back (Cortana did, and retconning a comic few people read isn’t worth losing a good story for). This was literally a Forerunner, and was so powerful that Chief couldn’t even fight him 1 on 1, but he’s just gone after one game? Chief beat the Harbinger easier than he beat an old Brute. How am I meant to take the Endless seriously if I can’t take their only representation in the story seriously?
See, I said I cared most about the story.
It’s just such a nothing story. 5’s was bad, but Infinite doesn’t have anything for me to like or dislike, because I have no reason to care. I felt more fighting against that random Brute Chieftain at the end of The Ark in Halo 3, because I hadn’t just had 8 hours of him telling me how great and important he was, when to Chief (me), he was just some Brute. The environmental story-telling of the Chieftain fighting Chief 1 on 1 with his Brutes watching made me care more. Escharum was definitely not on the level of Tartarus.
There was also no mention of Lasky; Blue Team; Arbiter; etc, beyond some audio logs. Great(!) Audio logs can work very well, like in BioShock, but they don’t here. They’re meant to be additional - not fill you in on the more interesting story that you’ve missed, and would rather be playing.
Succinctly, it serves only as an epilogue to 5’s story, and prologue to whatever is next. That’s boring. That’s what a short tie-in comic is for - not the Campaign itself. Imagine if Halo 3 was instead Chief waking up on the Forward Unto Dawn, and crashing into Requiem - with a random boss Elite spouting some stuff, and the Didact just making hollow threats about composing all humans, before dying. That’s Infinite. Escharum is Jul 'Mdama.
TL;DR: I’m not mad. I’m just disappointed.