Main series or just the first person shooters? ODST is definitely a spin off, Reach can be arguable. Anyways, Halo 2.

Five...because that is the one I played...

Though this will very likely change when I decide to play through all the main Halo games leading up to Infinite.

Combat Evolved, great game, mostly excellent level design, where even the weaker levels at least have a neat concept to them, the gunplay is cool as well as its combat scenarios, the music score is excellent, good pacing and story beats, is just a lot of fun, think it has areas where 2, and 3 surpass CE, but CE feels like the most well rounded and complete experience of the 3 games.

Bonus: None, also don't really feel like playing post Reach for now, so that is part of my none hopes.

Halo 3.

Bonus: More open, more focus on scope and sandbox gameplay. Hopefully not as an excuse to hamfist a battle royale mode into the game. As Shadow mentioned, much of what's killed the spirit of Halo was chasing esport trends with twitchy arena gameplay. Chasing battle royale would be just as bad a mistake.

I don't care as much about balancing if they can increase the overall scope of the gameplay. Halo has always felt like it could naturally fit a huge multiplayer mode with gigantic maps and vehicles to traverse them. Pelicans and Phantoms have been in every game but never truly usable. Elephants were roughly implemented and never really viable. Even something like the Shadow which only appeared in the campaign of Halo 2 used as a carrier for Ghosts. They already have a ton of assets available to naturally implement into a huge multiplayer mode. Just add a squad system similar to Battlefield to top it off. This has always felt like the natural evolution for Halo in my mind.

Of the games I played:

1. Halo 2
2. Halo: CE
3. Halo 3

1. Better campaign and multiplayer than the first. The addition of the Arbiter to the campaign mode was great, and duel wielding was fun. Net code was definitely better than the first game too. There was still a small issue with the host advantage, but only about half as much as the first game. The overpowered pistol was also gone.
2. Fantastic game to kick off the franchise. Played tons of campaign mode and tons of multiplayer in LAN with my friends in college. The pistol was very overpowered, and the net code caused some issues. We played each other so much, eventually we could tell who would win because of the advantage the host console had.
3. Don't remember a lot of specifics on this one. From what I can remember, the campaign wasn't as good, and neither was the multiplayer. I remember not liking the new enemy type they added. We didn't play anywhere near as much of this as the first two games. My friend group pretty quickly transitioned over to Gears of War 2.

Halo 2, best campaign and multiplayer. Spent countless hours beating the campaign over and over and eventually leading to the Legendary playthrough. Spent a lot time with couch multiplayer with my brother and friends. The disappointment of Halo 3 was only part of my overall dislike of the 360, but Reach was pretty good.

Halo reach campaign Halo 2/3 MP.

H3. H5 was the game with too much multiplayer and not enough story support.

CE, by a mile. It was a transformative gaming experience for me. I played a lot of video games from the time I was about 2 years old until my early teens (born in 1980, for reference). From roughly 2013 until the release of Halo in 2001, I wasn't gaming very much. But, my youngest brother and I played a multi-day marathon co-op session over Thanksgiving Day weekend. My girlfriend was out of town, so I had a lot of free time. We started on the Tuesday before, and played all night. My brother skipped school on Wednesday because he hadn't slept, so we played all day that day. This continued most of Thursday and Friday, and to a lesser extent on Saturday and Sunday. This is one of my most dear gaming memories, and an important building block in my relationship with my brother. I'm the oldest of four and he's the youngest, so our age difference in some ways meant we were not as close as either of us was with our other siblings. But, we've shared and bonded over a love of Halo ever since.

Also, it's just a truly great game. I agree that, strictly speaking, it did not establish the genre on console. But, I definitely think it was the most important game in terms of making the FPS genre into the dominant one that it became. Actually, I think it was one of the most important games ever made, in terms of its impact on the hobby.

Bonus: I too hope it is a bit more open. I think the series needs that kind of modernization. But, the most important thing is that it continues to feel like Halo. If the changes take away that Halo feel, then I'd rather it just remain a linear game forever. Even if that means it can no longer be a top-tier franchise. The biggest thing that Halo has going for it at this point is nostalgia. They cannot squander that by changing the feel.

Halo 3

Best gameplay
Best art design and textures
Largest levels
Best sandbox
Best weapons and balancing
Best story and cut scenes

It's probably my favourite game of all time.