Obstacle Avoidance

Hello, I'm working on a game project with a top down view of the game, where I'd like for the enemies to avoid colliding with eachother as they navigate the environment using complex flight patterns.

I've read a few tutorials here and there and have gotten very rough ideas at how it can be implemented.

Steering Behaviors with spheres is one, and raycast and sphere collision detection is another.

I'm trying to figure out the least expensive thing to do to achieve a simple avoidance for enemies. Anyone can provide me with a rough outline of what I can look for or what I should do?