I see citizens and cops as vital part of Half Life 2 RP, at opposite ends. The citizens are its life blood, there can be no roleplay without them, and they come in a variety of forms which makes every experience unique.
Cops serve the function of representing the oppressive force of the combine, VITAL in roleplay. Without them, all the fear and magic of living in this dystopia is gone. They are easily the most public face of the combine.
CWU and Vortigaunts are very good RP assets, serving their purpose to enhance RP and providing tools to achieve more. They are the salt that makes the whole more complete and enjoyable, while also being self-serving and allowing people to expand upon their characters.
CCA and Rebels. Now, these are only good depending on how you act on them. If the CCA is not very visible, it's as if it didn't exist at all, while if you play your cards right they easily become a focal point of events.
Rebels are either gung hos and rambos hiding in the sewers stockpiling on equipment and shotcopping, or they're actually very well developed characters that chose not to lie down and die.
I was not sure of what the first picture in C was, so I put it there. If it were loyalists, I'll probably place them down at a A tier as they are expansions of citizens and CWU.
OTA also goes to the C tier because they serve one specific function and are mostly flag on flag off roles. There is close to zero character development, but they serve the important functions of quelling down rebellions and being the thing everyone fears in case shit hits the fan. They're useful, just not very enjoyable to me.
For the D tier, I fucking HATE bird RPers. They're annoying, useless, a disturbance of roleplay at best. They add no real atmosphere, instead 90% of the time I see them being assholes and annoying players.
Headcrab RPers are overly zealous almost every time, attempting to PK people while forgetting they're there to play a P2L role. I cannot stand these two whitelists so I placed them here