Need Game Mod Advice Thread

Good day!

I'm a programmer by trade and have quite a bit of on-the-side experience working with Unity. That said, I've never modded a preexisting game before.

Cutting right to the chase: what first-person game capable of being modded is the most robust to work with--without having a reputation for being cumbersome--in your opinion? This game would need to allow for things such as "security cameras" that can show players what is happening in another part of a map. Custom maps. Custom graphics. Scriptable NPCs. Aircraft would be great as well. Combat should be able to include melee, non-lethal, and lethal ranged.

Your advice and input is appreciated. Yes, I did use search. I couldn't find a similar question that was recent (maybe I'm blind though).

Thanks, and if there are any questions you have that I didn't answer, let me know.