I chose Mascara. Choosing just one is difficult....
My eyelashes are pretty light blonde. I am naturally a redhead.
Mascara will instantly enhance my eyes and make them stand out so much. I rarely leave the house without wearing mascara.
For a natural look that is just the right amount of darkness and color, I usually use Bare Esscentuals Big Tease in black/brown. It has a neat brush- one side is long for great coverage and definition on your upper lashes; the other side is short and perfect for your lower lashes. Very natural looking, and slightly lengthening.
For more dramatic, I use Dior's Diorshow in Black. That stuff is intense, and allows me to do my 1960's doe eye look very well (coupled with liquid eyeliner, of course.)
My second most essential product would be concealer.