I've been a halo fan since they came out, been playing them my entire life and I was only 7 when I first played Halo 1. Until after halo 4 I've played every single halo game that has ever been released. So here's my experience/thoughts about each halo game
Halo 1: This is, IMO, my favourite halo game. It might just be nostalgia that makes me say this, but I know I've had and still have the most fun playing Coop with friends on this game. The amount of craziness you can get into, the amount of things you can do and the easter eggs you can have, it's fantastic. The only part of this game that is lacking is the multiplayer, while the multiplayer is very good on the PC (especially if you get the modded out Custom Edition) the multiplayer overall (on Xbox and what-not) has been improved in later titles.
Halo 2: When this came out I remember my friends and I thinking it was, overall, a disappointment. But when I revisited it, it actually is the most content filled halo game in the entire series. It has more enemies, levels and weapons than any other halo game, it's campaign is very long but it's overall story was a tad confusing and convoluted. Halo 2 introduced the halo franchise to the great wide world of multiplayer and with fantastic levels like Lockout it was edging to a great success.
Halo 3: While Halo 1 is my favourite, I can't argue that Halo 3 is by far the best halo game in the entire series and no game has ever scratched it's amazingness. With a thrilling campaign, fully fleshed out multiplayer, Forge mode, theater mode, arcade mode and a level of customizability never seen before in a halo game, halo 3 was quite simply a masterpiece and was the magnum opus of Bungie. It was a fantastic wrap up, the amount of secrets and hidden easter eggs in it, the amount of fun you could have developing your own levels and the competitiveness of the matchmaking, it's no surprise people still play halo 3 today.
Halo ODST: Perhaps jarringly, halo ODST is probably the second worst halo game ever made. It tried something new so that's commendable, but it overall failed in it's execution. Expanding halo into a more convoluted story you play as an ODST rather than a spartan, while this game promised radically different gameplay, with more of an emphasis on stealth and tactics, what results was basically a standard halo game but you die quicker and can't jump as high. Not really ideal, though Buck (played by the guy in Firefly) is fantastic and that first scene when you drop into new mombasa is fantastic. The multiplayer is basically Halo 3's multiplayer with only trivial additions.
Halo wars: Perhaps the worst halo game ever made, not to say that it was necessarily BAD, the actual game was good and it was made by the developers of Age of Empires. The problem with the game wasn't that it was bad, it was just that it was an RTS, on an xbox. Building an RTS for a console is one of the worst ideas I can think of, controllers are just not built for level of accuracy and quick swiping you need for an RTS. Maybe if they made it turn based, but overall halo wars was just not very fun, even if it did everything it could right.
Halo Reach: Halo reach was better than ODST and 2 but not as good as 1 or 3. It offered something new and was a good send off with bungie, adding some new additions to multiplayer and what-not, it was fun. Overall though, tbph, it was rather forgettable. It didn't really revolutionize the game in any significant way, it didn't advance the story and didn't really advance the game mechanics much. It was just basically tuning what was already there and while it does do a successful job in it, it doesn't stop it from being simply forgettable.
Halo 4: Halo 4 was... confusing. I didn't know if it was a great new start to the fantastic series, or whether it was a disappointing divergence that relied too much on current trending tropes in games like quick-time events. The game was good, and was pretty fun to play the campaign, multiplayer was ok but I didn't play much of it. I would probably put it right around halo 2, it was less forgettable than reach as it did change a bunch of things, but I think they've jumped the shark a bit with the story and the prometheams could have been funner to verse against. Also the scope and scale of the series seems dimmed, the Didact doesn't seem like as much of a threat as the flood, so it's just sort of a game that's there. I liked it but not as much as I wanted to like it.