Serum has the most concentration of good things (yep, anything, good thing, good stuff for your skin) so you have to wear your serum firstafter you cleanse your face so that your face could completely absorbs all the good things.
According to the packaging, again, your skin care routine should go in this step :
Serum - Skin (Toner) - Lotion (Emulsion and stuff like that) - (Essence) - Eye Cream - Cream - Oil
Or I have to summarize it, from the less moisturizing, to the most moisturizing.