In cod4 mw which is a better optic?

The red dot sights is the most effective in all weapons. The reason being the you clearly gauge the x&y positions of cross hair movements with a larger transparent mini frame than in ACOG.

The real minus of ACOG is it is quite similar to sniper scopes and ineffective for Assault due to heavy recoil and smaller mini frame(scope corner radius). So red increases accuracy and ACOG decreases accuracy.

The default sniper scopes give you greater zoom than ACOG so again it is a waste. What is the use of a sniper who cannot see long distance effectively?

However the red dot sights is real waste for a M16 assault rifle since it has lesser recoil. So Advanced combat optical gunsights are fit only for M16 no other. Check it out

It all depends on your situation. Normally, however, I would say the red dot.

one more vote for red dot

depends upon conditions as well