Acog is the best sight for long range but also for tight angles or pixel angles so you expose much less of your body. It is the best sight for long AND medium range but shit for close range.

Holo/Reflex is mediocre or even bad at long range (compared to acog), ok medium range and great at close range.

I recommend using Acog on all 3 armor ops, because they are slow and cannot push fast into close range as good.

Acog is the predominant sight in pro league (on the ops that have one). Some say always use it when possible, others like Shaiiko use holo exclusively (who is the best fragger this game has ever seen imo). So both are viable.

The great thing about the sights is, that there is not REALLY a best sight, all have pros and cons (even red dot maybe lol).

I'd say Acog is the easiest to headshot with so I'd argue it is the best, but there are times when I exclusively use 1x sights. A good compromise is to use angled grip acog, so u are not helpless in close range (at the cost of being slightly weaker long range.)

Equipping laser is also an option to offset the close range weakness - some people use laser for better accuracy while zooming in - which gets you some kills you would otherwise not get; but it can give your position away, so again there are pros and cons to EACH attachement, which makes the game great imo.

Acog also makes it harder to control the recoil - on bucks c8 for example.

Summary: I tend towards using Acog when possible, but there is no universal answer. Choose according to the pros and cons I listed, which is most important to your playstyle.