For Campaign, that's easy: Halo CE. It still has the most fun campaign to date. I like the level and encounter design and the difficulty is well-balanced, with Legendary being challenging but not cheap like later games (esp. Halo 2).
If I had to rank all the Halo campaigns in terms of gameplay/fun factor, it would be CE>Reach>ODST>3>5>2>4.
For multiplayer, that's a tougher one. I still have a lot of fond memories of all the CE LANs I attended from 2002 to 2004, but playing that game online revealed how broken its weapon sandbox was in a multiplayer context. Halo 2 had arguably the best maps in the series, but it too suffered from serious weapon balancing issues, mainly deriving from dual wielding, the sword having unlimited energy, the plasma pistol charged shot having excessive tracking, the shotgun being wildly inconsistent, and power weapon not respawning on a timer. To this day, I consider Halo 2 to have the most utterly broken gameplay of any Halo game's MP, which is a shame considering how great its maps are. Halo 3 fixed a lot of the issues present in Halo 2, but it introduced issues of its own, namely the weapons being way too mushy and inconsistent thanks to reduced aim assist and the lack of hitscan. Reach's overall sandbox felt better to use than Halo 3's, esp. with hitscan returning, but it introduced armor abilities and Bungie decided to have those as part of preset loadouts rather than on-map pickups, which produced all sorts of other balancing issues. Of the Bungie-era games, I'd say Reach was the best.
Moving on to 343-era Halo, the multiplayer has continued to see improvements in many areas but again those improves came with changes that detracted from that. Halo 4 probably had the best weapon sandbox at that point, but it retained armor abilities (replacing the ever-annoying armor lock with the just-as-annoying X-ray vision ability) and took the loadout ball and ran with it, giving players full customization complete with CoD-style perks. Halo 4 also largely replaced traditional weapon spawns with ordnance drops, with personal ordnance being Halo 4's rough equivalent to CoD's killstreak rewards. Halo 5 I think has the best overall weapon sandbox in the series, with everything being useful and having a purpose. BR/DMR starts were greatly de-emphasized, meaning the constant cross-mapping in previous games wasn't nearly as big of an issue. But on the flipside, the Spartan abilities greatly accelerated the pace of combat. The more deliberate, methodical pacing that had defined Halo was replaced with a high-speed, high-mobility, and therefore far more "twitchy" experience, and it was hard for my old ass to keep up with.
Halo 2 Anniversary, despite not being part of a main series game, was probably the best-playing Halo multiplayer to date. It's basically close to how Halo 2 should have played back in 2004. The only bad thing about it was that only a handful of Halo 2 maps were remade for this mode. Where's Midship, Waterworks, Terminal, Turf, Ivory Tower, etc.?
If Halo Infinite can keep 343's good weapon balancing, slow down the pace of combat, and make any mobility-enhancing abilities tied to on-map pick-ups (which seems to be the case with the grappling hook, at least), it could end up being the best main-series Halo MP to date. But at the current time, I still don't think I can make a definitive ranking of all the Halo MP modes.