Not the worst at all, and not sure there is a worst as all mainline games are quite good. It’s however the one which is most different from the previous formula, due to its open world elements.
To me the open world was both good and bad. I loved exploring the map and getting a real feel for Zeta Halo. And I also loved having wee skirmishes with foes all around this world, taking down outposts and having the freedom to experiment with different weapons and vehicles. On the other hand it really broke up the narrative, I felt like all the big plot points were at the linear sections in the beginning and ends of the game. And as someone who loves exploring to death it felt weird not hearing the characters much for big portions of the game; so I think any future game using open world would need to try and reconcile this.
In terms of storyline, I don’t think it was as good as the Halo 1-3 war setting, but it had potential to go to interesting places if it ever gets developed further. Time traveling Endless could work well if the concept could somehow be implemented into gameplay (e.g. Marathon Infinity), but they didn’t really do much in this game. The Banished were also more interesting than [insert generic covenant fodder remnant of the week] that were being fought beforehand.