I mean, I still love CE’s campaign - even tho I can see areas where it’d really suffer for anyone who didn’t play it way back when, particularly in how the levels can be so easy to get lost in sometimes - so I’m maybe not the best person to answer this… But you might want to try Reach? It’s a prequel with an almost completely separate cast of characters, so you won’t feel like you’re stepping into a story halfway, and I think the level design is generally better thought out, even if I’ll always have more affection for CE’s big confusing boxes. (Less so the massive networks of corridors.)
Actually, wait, shit, I just remembered that Reach isn’t in the MCC yet. Uhhhh… ODST is super good, and similarly standalone, so maybe try that? It’s not exactly a ‘traditional’ Halo experience, though.
I personally wouldn’t recommend playing Halo 5, even though I generally like the basic gunplay/movement, because the campaign is - in my opinion, at least - absolutely terrible. The story’s nonsensical and incredibly poorly-told–like, there’s some ideas in there that could’ve been interesting, but it’s pretty much impossible to get invested in anything that’s happening because the game never takes the time to show you who anyone is, or why you should care about what’s happening. And, even ignoring all that, I found the actual missions mostly boring–and there’s this one boss fight they make you do, like, four or five times, with basically zero changes–and it really wasn’t much fun the first time. Halo 5 is the only Halo game I outright dislike.
Halo 4’s campaign is significantly better, buuuuut the campaign story relies heavily on you having followed the expanded universe up to that point, and it also utterly squanders the potential of Halo 3’s ending, in terms of what they could’ve done with those characters and that universe going forwards. (Which, I’ll admit, is probably a slightly unfair criticism to ding the game over.)
Halo 2 and 3 are both pretty great in their own ways, but whether you’ll enjoy them really depends on what it was, specifically, that put you off CE. If it was the level design, then both are generally better - or, at least, more focused - in that regard.
Overall, my firmest recommendation is definitely still Reach, but you’ll have to either pick up a cheap copy - it’s on Xbox One backwards compatibility, but not Game Pass - or wait for it to be added to the MCC/Game Pass, which should be sometime later this year I wanna say?