Sorry in advance for the essay
While I hate the majority of what Reach has to offer from a multiplayer perspective, there are two modes that I absolutely and totally adore to no end. Those two modes were the sole reason I kept playing Reach for 8ish years.
ZBNS (MLG)- I love the -Yoink- out of this game mode. It felt like what I wanted Reach to be from day 1. The DMR itself feels like the hitscan BR from Halo 2 (albeit a little easier to use), but the strafe mechanics in this mode allowed for some really interesting in-game DMR battles. I remember having to re-learn strafing when this mode dropped since it wasn't a necessary skill to have in Reach MM beforehand. I also love the original forge maps for this playlist, even though most people probably despise them. I felt like there were some really cool competitive offerings in this playlist gametype wise. Nexus KOTH comes to mind as a really unique map design for KOTH, since we didn't really see a map design like that in Halo prior to this playlist. It felt like the last real exploration into H2/3 era gameplay that this series has seen. Like they took the foundations of 2/3 MLG settings and tried to craft new experiences with that same formula. This mode also worked amazing with the maps in the CE Anniversary DLC. Even though it doesn't hold a candle to H2/3 MLG; I still loved it and can't wait for it to return as a ranked offering with Reach.
SWAT- I couldn't stand SWAT in the previous halo games and I most definitely hated it in 4/5; but ironically it was one of my favorite gamemodes in Reach by far. Since the map design revolved around sprint in Reach, it sorta ruined many of the maps for traditional slayer. However, somehow, the map design really played to the strengths of SWAT as a gametype. Since many Reach maps featured elongated corridors to account for sprint, SWAT's 1-shot-headshot gameplay really shone in these corridor-battles. The gametype was all about strategic positioning, predicting spawns and being able to read where the enemy was at all times. Furthermore, SWAT remedied many issues with the map design in Reach. It was a lot easier to break a setup at top yellow lift on Sword Base when one single player could lift up and outgun three enemies for the triple kill. Setups swapped hands back and forth over the course of the match, something you don't really get to see in regular slayer in Reach. And finally, bloom actually served a purpose in SWAT and was the only gamemode in the entire game where bloom didn't destroy the shooting mechanics. Where bloom was a game-breaking mechanic in slayer, it almost added to the tension of gunbattles in SWAT. With bloom, it made it extremely important to land your first shot as the headshot because if you missed that first shot, you were punished with inaccuracy and randomness on the second shot. It rewarded skilled gameplay since you had to hit your first shot, and it punished bad aim because it was a lot harder to win a gunbattle when you had to account for bloom and the enemy doesn't. Overall, it was a unique and interesting experience that we haven't really seen implemented in the same way since Reach (4's DMR and BR diminished some of the complexity that SWAT had to offer imo). It will also be great to experience this on dedicated servers since some matches on 360 turned into a connection battle with someone from another country, and hurt the gameplay significantly.
Overall, both of these modes were such unique experiences for me. I personally never played anything like them before Reach, and I haven't played anything like them since Reach. And while I may never play any other gamemode from Reach, these two are all that I need to get thoroughly hyped for Reach on MCC. Sorry for the essay everyone, I just absolutely loved both of these modes.
TLDR; MLG and SWAT