There is actually a lot of benevolence in the Star Trek galaxy.
The Orgainians can conquer all to combined empires we ever see, but they just want to be left alone.
The Metrons might be able to as well, but they don't really leave their system.
The Talosians are less "benevolent" but they show the ability to project convincing illusions over light years distance and could certainly carve out quite an empire with that ability.
The First Federation really didn't seem aggressive, the Fesarius seemed like it could conquer quite a bit, and we don't know how many there are like it.
The Thasians could be very dangerous, but they only seemed to be concerned with other's saftey, saving Charlie and then saving everyone else from him.
Trelane wasn't benevolent, but his parents were. If their "kid" could create planets and drive them around, I think they would have serious power potential.
I haven't even gotten out of the first season, but I think I made my point.
And I agree, the Federation seems exceptionally "nice" to almost everyone, the Haulkans, Capellans, and Organians (before they reveal their true nature) case in point.