Arch Ville, along with the Revenant, are the two most important monsters added to Doom 2. Ironically, they were the two monsters removed from Doom 64, and one of the reasons why that game sucks.
All the additions to Doom 2 were important, actually. Before Doom 2, all enemies that fired projectiles worked the same way, with projectiles aimed at the player.

Imp: aimed projectile
Caco: aimed projectile
Baron: aimed projectile
Cyber: aimed projectile

With Doom 2, the monsters added depth to the game, and takes it from something shallow (doom 1) to something complex (doom 2):

Revenant: melee, targeted projectile, guided projectile
Mancubus: projectiles that spread
Arch Ville: magic attack
Pain Elemental: projectiles are actually enemies

The Hell Knight, mini spider, and heavy weapon dude don't add anything revolutionary, but they are also important and add depth nonetheless.

I can't understand Half Life. I like Doom and Quake and Serious Sam and those games where you can jump into the middle of 50 enemies and with the right movements, prioritization of targets, and skill, you still come out victorious, but I play Half Life and a guy starts shooting me in the face and I shoot him back and I only win because I have more HP than him.
I even tried to understand the logic of the game, like throwing grenades to force enemies to move and while they're walking, you shoot them, and they keep walking as if nothing happened until suddenly they fall like shit on the ground because their HP reached zero, but I think it's all super lame and stupid.
I guess people who like more tactical stuff might like it, I don't know.
For me, Half Life 1 and 2 are typical Western games: kinda terrible gameplay, but focused on the interactive experience, immersion, simulation, etc. Everything Westerners love, and one of the reasons why they still don't know how to make video games.