And this year’s the blue ribbon goes to…
Persian Speedwell (Veronica persica)!

Granted, the first signs of spring depend on where you are when you notice. I was walking my 45 minute trek home from school one afternoon, passing the outskirts of a city park when I saw familiar blue faces feathering the sidewalk edges. Later while passing a university hospital I looked up to notice the dark rouge buds of the soon-to-bloom red maple (Acer rubrum) trees. The first sign of spring among woody plants I suppose.
Grasses cheat though. Since it is still technically winter (despite some warmer days where I live), I do not know is the occasional ground greenery are from hardier, cold season grasses that have been around for a while or from spring newbies.
I dare you to get a dichotomous key for flowering plants, a dissecting microscope, a fine cutting instrument and a narrow probe. Pluck a grass that has grain stalks. Peel back the minuscule florets that hide the grains. Slice the grains evenly in half. Assuming you have amazing motor skills with your hands and did all that properly, follow the key to identify the species. You probably won’t, because grasses are extremely diverse and notoriously hard to identify.
And if, for example, you know what fescue looks like, well you are not done. Fescue is a genus with hundreds of species.
And a lot of them grow all winter where I live, so they do not count as signs of spring.