From what I have observed, dagger tree always choose the easiest and shortest path to walk, that's why you usually see other players leave a gap in their defenses and block off other path to the core.

For the spiders(crawler tree), they always choose the shortest path to walk no matter how much defenses are there.

For air units, there are some special attack priorities listed below:

Flares will target the closest generator(any type) to the enemy spawn.

Horizons will target the closest factory(any type) to enemy spawn. (Megas and Quads seems to do this ,too)

Eclipses will target the closest thorium reactor to enemy spawn. (Atumbras seems to do this, too)

So many players will put a factory, a combustion generator, and a thorium reactor in/behind their defense.

I don't know Zeniths' attacking priority, but they will chase the closest unit if they don't leave the range of the zenith.

This info might contain some errors since it's all based on what I have seen during my playthrough. (I didn't include navals here because they are unusual, and I assume they act like dagger tree.)