Really curious and interested to see the communities preference of the visuals that the covenant ground forces had donned over the decade. Halo 5 guardians is in this as a mystery element, nonetheless let the voting begin!
Edit: Just to clear things up, I’m not talking about the graphics engine exactly, I’m talking more about the actual designs such as armor and physical stature.
I’m torn between H2A and H5 mostly because, well, they kinda look the same in terms of design.
Halo 3 and ODST, whenever I think “Covenant” I picture the Halo 3 look for the Covenant in my mind,
Halo 1 and Halo 2
Very torn man. Hard to pick, especially considering each game pushed graphical/art design to new levels.
I didn’t really like halo 4, but they had he best graphical design nearly for everything.
It’s Halo: CE for me. While vehicles (especially the Wraith) have gotten way better with the newer games nothing beat those Halo 1 elites. Gotta love that angular design (yeah I know they look like that because of the technical limitations of the OG Xbox)! Spec Ops elites in particular looked freaking awesome.
I have to say Halo 2A because of the fact that they were able to do a detailed version of not only the armor, but the alien as well.
Halo 3 the Covies looked their best, the grunts had changed up their look, while the elites looked similar but updated. Brutes also donned heavier armor and lost the Honor guard gear. My preferable covenant look
I would’ve loved halo 2 anniversaries if the grunts weren’t in the HR design. I prefer 3/ODST
I would say either Halo: Combat Evolved, or Halo: Reach. The both of those have my favourite Covenant designs. I’d also like to mention Halo Wars, although this would have to apply to the cutscenes. 
Halo 3 because its much cleaner visuals finally showed off a gorgeous Covenant. All design evolution after that was just gravy. Halo 3 is what really set the table for me.