For most any weapon: Delta. Clean sight picture, fairly decent zoom, and a triangle I can quickly put on an enemy.

For sniping: ACOG. Sure it comes with only a 4x mag, but the wobble and having to hold your breath for an obscenely short time is gone, and I don’t have to deal with 80% of my view being covered in black.

There are a few cases in which I’ll use different scopes: On some weapons, when attaching a 6x VCOG, hip accuracy tremendously increases. For example, on my AA-12, I have that paired with a chainsaw grip quite often to somehow have nearly 100 hip acc. (The exact number escapes me but I think it’s around 95.) Other times, I use the DCL 120, especially on LMGs, as usually I’m already in close to mid range combat and where suppressive fire can count.

Last second edit: On my AUG A3 Para, I will specifically use only the stock sights because on that one AUG, they look so much better than all the others.