I usually commend in this order. keep in mind I play tank.

24 man content. Most impactful healer, most impactful dps
8 man content. Co tank, most impactful healer, most impactful dps
4 man content. Healer, unless the dps does something impactful.

Generally I only commend the dps if I am playing as a dps or if the dps does something impactful like raise, or if they are a dancer and make me their partner the commendation automatically goes to the dancer.