Great question, OP...but I REFUSE to pick just one!!
I agree with lovemysavior -- the new Tone:Grey quad is stellar, I just got it and have already worn it several times. Every single color is highly wearable and they blend so nicely together.
I also Violet Trance, Vanilla, Bamboo, Rule, Beautiful Iris, Sable, & Unbasic White.
Ask me tomorrow, I will have some more faves. This is the best brand for e/s (at least for me).
Carbon. I wear it pretty much every day as a top liner. I wear UD Yeyo under my eyes and the fallout from the carbon makes a nice, smudgy bottom liner in a silvery color. Does that make sense?
My absolute favorite is Aria (although I'm almost out of it and it was LE from a couple of years ago and I can't find it anywhere!) and also Henna (also LE, but I got smart and ordered a bunch of backups).
These are both my favorite shades to use especially on mornings when I'm running a little late and still need to do makeup.