Okay, I had just played through Half-Life and OpFor recently and experimented with the different weapons on certain enemies. Now if were possible, I would use the Gluon Gun for every scenario, but there’s a scarcity of ammo.
I found alien weapons such as Hivehand and Shockroach to be horrible against there users. Other than that the Shockroach is extremely effective on other aliens and grunts. Hivehand sucks, hands down. Best alien weapon would be the Spore Launcher. Great for every situation. In HL1, Snarks were the best weapon for handling grunts when I had low health. Perfect distraction for me to get past them all.
So peeps, post your preferred weapons and selections for handling certain enemies.