I wish that someone had warned me that one of the most important aspects for picking a system to run is how difficult or complex it is to create NPCs or monsters.
I ran D&D 3.X for a time, which... wasn't great for that.
Then I ran Exalted 1E and 2E, which were worse.
These days I mostly run D&D 5E, which is (a) vastly simpler when it comes to NPC prep, and (b) has so many stat blocks in both official and unofficial sources that I rarely need to come up with something custom.
A honorable mention goes out to GURPS, which is actually pretty easy to run for once you know what you are doing - first you need to keep in mind that "character points" are mostly for player characters, and can be ignored for NPCs for the most part. Then you also need to keep in mind not do overdo it with defensive stats, or else combat will get bogged down and boring.