Greetings Spartans!
I posted this in another thread, but since I can’t delete comments, the whole thing looked like a mess, so I’m reposting it here. I also add much more, so if you already read it, take another look 
Bold words are HUD element that are consistent with Reach, since that’s the standard that has been mostly untouched over the last decade and it made good improvements over H3’s HUD which it used as its base.
Halo CE: Weapons (top left, oddly not above your weapon model, secondary weapon not visible, analog ammo), Health (Top Right), Grenades (top left), not all are visible, can be switched with a button press, Radar (bottom left).
H2: Weapons (top right, fixed from CE, & top left for dual-wield, gun model on both sides, secondary weapon visible, analog ammo), Grenades (top left, all are visible, can be switched with a button press, Health (Bottom left…), Radar (also bottom left), Active Camo (Top Left, next to grenades).
H3: Weapons (top right & top left for dual-wield, gun model on both sides, secondary weapon visible, analog ammo), Grenades (top left, all are visible, can be switched with a button press), Health (Top Mid, prominent part of screen, very close to reticle and weapons), Radar (bottom left), Equipment (Top left, under grenades, both are consumables that the player tosses which is why it’s ok for them to be close together).
ODST: Weapons (bottom Right, close to weapon model, secondary weapon not visible, analog ammo), Health (top mid), Grenades (bottom left, all are visible, can be switched with a button press, no radar so space is free real estate). Most halo games have modes that remove the radar (slayer pro, fog skull, some infection modes, etc), making the radar the least important element.
Reach, the standard: Weapons (top right, secondary weapon visible, secondary weapon ammo number visible, analog ammo), Grenades (top left, all are visible, can be switched with a button press), Health (Top Mid, prominent part of screen, very close to reticle and weapons), Radar (bottom left), Armor Abilities (Bottom left, over radar, replaces radar in radar-less modes).
H4: Weapons (top right, secondary weapon visible, analog ammo), Grenades (top left, all are visible, can be switched with a button press), Health (Top Mid, prominent part of screen, very close to reticle and weapons), Radar (bottom left), Armor Abilities (Bottom left, over radar, replaces radar in radar-less modes), Ordinance (Bottom left, over Radar, not in most modes).
H5: Weapons (top right, secondary weapon visible, secondary weapon ammo number visible, analog ammo), Grenades (top left, all are visible, can be switched with a button press), Health (Top Mid, prominent part of screen, very close to reticle and weapons), Radar (bottom left), Thrust recharge icon is located near health, but it recharges every like 3 seconds, so you almost always have it.
Infinite: Weapons (Bottom Right, close to weapon model, secondary weapon visible, secondary weapon ammo not visible, no analog ammo), Grenades (bottom right, not all are visible, can’t be switched with one button press), Health (Top Mid, prominent part of screen, close to reticle, far from weapons), Radar (Bottom Left), Equipment (bottom right, next to grenades, both are consumables that the player tosses which is why it’s ok for them to be close together).
Summary:
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Weapons got 2 big additions over the years. Halo 2 moving them to top right to be above the gun model, and Reach showing us how much ammo our second gun had. - Grenades had 2 big changes, the separation of them from weapons. Keeping them on the left aligns them with Chief’s left-handed throws and treats them like consumables, just like equipment. The second was visualizing all grenades at once, if the slot was empty sometimes. - Equipment had only 1 change over the years. Reach moved AAs to the bottom left. This was a good change since AAs are not consumable like equipment and grenades. So separating them implies a difference. Infinite is bring back equipment and thus, they have been reunited with grenades. +1 for Infinite! - Health had 2 changes. H3’s centering the health from the corners that CE and H2 set up, ODST’s use of stamina. Only the secondary health bar was visible. Stamina, ODST’s equivalent to shields, was represented by a “bloody” screen, which be came VERY prominent when close to death. - The motion tracker has never moved with the exception of ODST’s removal of it. It is also the least important element.Overall consistencies:
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Weapons were always at the top, eventually aligned with the gun model on the right (also DW), analog ammo since CE, and for the last 10 years we’ve known how much ammo our second gun had. For the first time, ammo will not be analog, meaning ammo can’t be read with peripherals. - Grenades have been untouched since H2. Now Infinites cramming them in the bottom, and requiring a sub-menu navigation, despite the fact only 2 grenades can be held at once. Also, all Nades can’t be seen at once, eliminating peripherals again. - Equipment is meant to follow grenades, which they do in Infinite, but they’re crammed with weapons again which were separated after CE. This is simply information clutter. - I doubt health will ever leave top mid, but it is now very far away from the rest of the important info. - Motion tracker: See above.
Intermission: If you think this is nitpicking… yeah it kinda is. But also consider: How many days of gameplay do you think you’re going to spend looking at this HUD for the next 10 years? The HUD is probably the single most important thing in Infinite’s UI.
Most important HUD elements IMO:
- Health (Reticle is always center so doesn’t really count) - Weapon Ammo/Model - Consumables (Nades and Equipment) - Score - Feed/Medals - RadarThe Health Bar having close proximity to all offensive indicators (Reticle, Weapons, and Consumables) means your eyes don’t have to travel far to find the most important info. The weapon model is far away, but it is big and the silhouette makes it easy to see in peripherals. Also logic reasons

Score and feed can honestly go wherever, but feed is traditionally on the left side. Also score probably shouldn’t be too close to the feed. Finally Medals should be next to the feed. Seeing an overkill medal and the feed in one glance is just smooth IMO.
Conclusion:
It seems to me that 343 was trying to go for the more “modern” HUD that you see in COD or TF|2. But Halo’s slower, more methodical gameplay forces more info to be important. Health doesn’t matter in most shooters since you die in 2 bullets. As a result, neither does ammo, really. Also most games either give you unlimited ammo (TF2), or you are very unlikely to use all your ammo before dying (COD). And neither really encourages you to pick up fallen guns. In both examples, Grenades don’t matter. You can’t pick up other grenades, they usually have a cooldown, and in the case of COD, you can use lethals and tacticals at the same time, so no grenade switching.
This is all objective fact and should be seen as the absolute truth. And if you disagree, you are W R O N G.