The most dangerous threat at the time.
If I'm low on health and a single hit from dying, well then take care of the puny things that can run up and kill you quickly.
If I'm not low on health and can deal with small attacks for a bit and there is a large enemy who is far more dangerous, I'll take that out.
If it's a diaper wearing man-baby with bombs for limbs running at me, then I'm playing Serious Sam and it doesn't really matter.
If I'm low on health and a single hit from dying, well then take care of the puny things that can run up and kill you quickly.
If I'm not low on health and can deal with small attacks for a bit and there is a large enemy who is far more dangerous, I'll take that out.
If it's a diaper wearing man-baby with bombs for limbs running at me, then I'm playing Serious Sam and it doesn't really matter.