My favorite is Sunday Riley CEO, though far from affordable. I’ve tried the Drunk Elephant and the Ordinary and this one still wins for me. There’s a new affordable brand called the Inkey List that has few ingredients and I purchased the Vitamin C from them for $15 last week but I haven’t tried it yet. I’ll keep you posted on that one!
I guess it depends on your definition of affordable. This one is $90 but using only 1-2 pumps per application it has lasted me quite a long time. I love it because of the high amount of bio-available vitamin c it has. Marie Veronique Ferulic Serum, I've been using it for several months now and still have tons left and my skin has never looked better. I've used so many others but this is my new standard.
If you're not looking to spend big money, I highly recommend L'Oreal's "AGE PERFECT" Hydra-Nutrition Facial Oil. My whole family has been using this product for a couple of years. It's literally something we put in stockings for Christmas. You can buy it at CVS.