Early Game
Grenadier is great for bomb damage and cover removal. Reaper is perfect for field commander and chosen assasin. Skirmisher is a beast early against advent. Ranger is always good due to very accurate damage. Templars lacks reflect-deflect early and is too risky. Sniper and specialist have low damage.
Credit to https://xcom.fandom.com/wiki/XCOM_Wiki for the images.
