In my opinion, Halo: Reach still stands as the best Halo game to date. Here's why:
Multiplayer: Halo: Reach's multiplayer introduced new, exciting game modes and armor abilities while also sticking to the game's core elements. (Now, I LOVE he armor abilities, so I think those should stay for the next Halo game.) The map design was phenomenal and no map was the same. The weapons were perfectly balanced (like Halo 5's).
Firefight: Firefight is a game mode I really miss. When MP got boring, I didn't have any Forge ideas, or campaign was already finished, I would revert to Firefight. The endless round and wave based gameplay was a blast, especially with friends. I think 343 should consider bringing Firefight back.
Forge: Halo: Reach Forge was incredible and it's only getting better, so I have no doubt it will be amazing in the next Halo game.
Customization: This is probably the second most important. This also coincides with the way you level up and acquire these items for customization. Being able to customize each individual aspect of your spartan's armor was a blast and I really miss saving up my credits to buy some specific armor piece that j wanted. I hope the leveling system and customization in the next Halo game are more like Reach's. (Not a huge fan of customization options being only in REA packs, but I do like the vehicle and power weapon aspects.)
CAMPAIGN: This is by far the most important item on this list. Reach' campaign was devastatingly emotional and incredibly action packed all at the same time. The music, camera angles, missions, and basically everything else in the campaign was absolutely gorgeous and I never had a second thought about replaying a level. Reach had an amazing story and I feel the Halo games have been lacking that. (I'm pretty fluent in the lore, so that's not a reason. I'm saying the actual story in the game and how you feel towards each action and death.) The campaign was so emotional and strong and it was the first time you really felt hopeless playing as a spartan. It was a good change.
Those are the reasons that I believe Halo: Reach was and still is the best game in the series. I hope 343i takes some of these things into consideration when planning for the next Halo. Let me know what you guys think!
-ROY G BIV 19
PS: this took me a while to think about so please take the time to read it <3 (:
Below is all my honest opinion.
Multiplayer: I'm not a fan of armor abilities at all. I don't want them back with reach and If we had to have abilities of any kind, id rather they did it the way they did with 5. The maps for me where terrible and ripped right out of the campaign. The only map I kind of liked was the spire.
Firefight: I loved firefight and would like to see it return.
Customisation: I also agree that the way they did it in reach was the best. I don't like the req system in halo 5 at all because it all comes to luck. I don't like how there is no skill based rank system tho.
Forge: This was also the best in halo reach for me. However if forge in halo 5 works the way it's supposed too, then reach will drop to second.
Campaign: I'm sorry but reach had the worst campaign to me. The story completely ****s up everything already established and completly S***s on everything the fall of reach already established and completly ****s up the lore since 343 treats both reach and the fall of reach as cannon, I couldn't get attached to the characters because they weren't developed enough to me and I just shrugged off all their deaths. Say halo 5's story sucks all you want but at least it didn't screw up the cannon. The gameplay is ok but not as good as other halos. The only missions I find myself replaying are the sabre one and the final mission where you survived as long as you can.