These are some of mine
Some of the Gundams like the Barbatos or the RX-78
the iron man and war machine armors
samus armor
armors from DC like Batman justice league buster
some transformers themed armor like optimus prime in age of extinction after he gets the sword from lockdowns ship
Some the power rangers mega zords and the red rangers powered up modes(I have some favorites but I would have do some research and find my old favorites)
the pacific rim Jaegers
star wars clone trooper armors and the mandalorian armors
I think the death troopers from star wars might make a pretty cool set of spartan armor. Samus’ armor could be cool in halo if it weren’t for the giant shoulder pads that the varia and gravity suit are known for, but there’s a few versions that would work like most versions of the power armor or any version of the PED suit from prime 3. On the topic of metroid, the federation marine armor would also look nice if it were adapted into the halo universe.
Mechagodzilla, Predator and T-800 themed armors.
Some titan armor from destiny 1, it wasn’t to bulky back then and would look good on a spartan, especially the one form the taken king live action trailer. Lord shaxx would be cool too. Doom guy would work as well
New Praetor suit from Doom Eternal for the Doom Slayer. If we’re going to talk crossovers, we have to go all in 
While Anthem by EA was a Nice game to Play once or Twice, The Suits were really Nice especially the Hulk Buster Style That shield would bust through grunts like a white hot Knife through Butter. And well Destiny has a good collection or Armour styles.