If I go with the monsters I use the most as adversaries for my players I guess I'd have to say humans? They're the most versatile monster in the monster manual IME.
If I have to exclude humans and go with something a bit more visually interesting probably illithids. I actually really like Brain Collectors from way back in the day - BECMI didn't have illithids but it did have Brain Collectors and I had one who was a mastermind behind the scenes for a campaign. But illithids are almost as good.
If I have to exclude humans and go with something a bit more visually interesting probably illithids. I actually really like Brain Collectors from way back in the day - BECMI didn't have illithids but it did have Brain Collectors and I had one who was a mastermind behind the scenes for a campaign. But illithids are almost as good.