H5 falls to my second. I played that game so much. I hit 152, #5 champ in solo q Arena, and top 10 champ in a few other playlists. I loved that game so much. So much, that it drove me to insanity to delete my account.... As I come back to Halo though, I think H3 deserves the "most competitive" Halo spot. To this day, the only people playin Hardocore H3 Arena or Dubs are only players that've either played sweaty since 2007 or pro players. Kinda same for H5, but H3 will prolly live longer and has a better reputation.
Halo 5 was designed for competitive by an actual pro team and the old halo 3 pro players that play 5 pretty much all agree 5 is the most competitive. Halo 3 was dope for its time though. Lots of fond memories back in the day.
Halo 3 has a better reputation predominantly due to it's social and casual aspect. Remember, the majority population hands down belongs to the social players playing the social playlist. 5 was a disgrace when it came out....and to top off the lack of social playlists...what was available was ruined by people getting the kill commendations for the Achilles set in warzone, and even btb was a ranked playlist back then. No social playlists for slayer or team objective....and now what do we have? Quick play and the random core play that comes in? Triple team and roaming king every now and then? These rotational playlists are more than likely playlist players would have wished to been there at the start...but the population is too small...but even then I would still argue that halo 5 is just to fast paced and in your face. It isn't like 3 where there's essentially 2 speeds in the game causing the game to play quite differently between the default settings and the competitive settings. Its just 1 speed across the board.
Compare to halo 3...I believe even the ranked playlists had somewhat of the social settings. The only real true competitive setting playlist was the MLG playlist which had the modified settings to speed the game up, higher damage, BMS, and modified shield recharge rate. So what people remember halo 3 for (the people at large and not the competitive community) is the slower cadence of battles and all the crazy fun they had in that game. Halo is after all at the end of the day played for fun...and man...I've been in these forums since the beginning. Let me tell you that I've never seen so many users angry at a halo game, extremely frustrated etc.
So although I love 5, agree with OP and everything...halo 5 has this hump a player has to get over to enjoy it and basically understand it. I feel like so many players left without giving it a real chance. They couldn't take the game and in many ways the game had no real place for them to play vs previous halo games. (Plus there are other reasons people quit. The entire ranking system was broken all the way up to 2018. Very inaccurate rankings. There are players that are platinum level today that have ranks from back then in champ tier and it shows champ as their all time high. Some of those players left immediately once the only means of keeping the inflated rank was to have some insane win ratio of like 96% or some high 90s. Who the heck can do that...so each time they got one was basically to derank so they said screw this game.)
Halo 5 holds a special place in my heart though. After a handful of uninstalls...I still play it with my homies today. Such an addicting game...and I don't believe there will ever be a game to feel like 5 ever again.
Why did you delete your account btw? That's a weird thing to do.