I saw someone say ODST had the best firefight, I thought that was easily the worst of the firefights myself haha
They're different enough that it comes to preference.
ODST: Tough as nails to survive for long, requires constant communication and teamwork. Maps, spawn areas, and weapon spawns are crafted to support this design philosophy.
Reach: Tweaked drastically to make it work for matchmaking. Now you have general loadouts, which generally focused on getting the highest score possible in a time frame or completing one set, rather than the 4 sets generally aimed for in ODST because of Endure. While you could bump the difficulty to be more like ODST in custom games, it functionally became a blow off steam mode. The hardcore mode basically became impossible to play as the low stress Arcade mode became popular.
Halo 4: Replaced with Spartan Ops, a failed experiment to give story players something worthwhile in the matchmaking side of the game. The story was bad mostly from virtually all characters being terrible, especially Fireteam Majestic that were the focus of the cutscenes while your own Fireteam Crimson was there to clean up their messes. Overall I'd compare this more to the campaign gameplay than Firefight. Other than having way more enemies it was pretty much the same, with weapon drops and was no round system. Outside of gameplay this is the example I usually go to for 343 clearly wanting to but their own huge stamp on the series, with the heavy focus on how "cool" the Spartan IVs were, something that didn't actually have any merit until Halo 5 with Fireteam Osiris.
Halo 5: Added as an afterthought to try and get Arena only players to use the weapons you can only get from microtransactions. It was ridiculously hard on launch, trign to get you to burn through REQs to have any chance of finishing. Once they added more difficulties it became much better. The easiest mode, Heroic, feels between ODST and Reach, though a lot closer to Reach.
When you look back at ODST, and remember that it was made by the core team that then went on to work on early Destiny instead of Reach, it's pretty clear that a lot of Raid ideas were taken from it. Possibly the Nightfall to, with ODST adding the Skulls in as you got farther.