I think the best Valentine is a poem.  Preferably one written just for you.  The little boy in this book by Cynthia Rylant (illustrated by Fumi Kosaka; HarperCollins, 2005) writes a simple Valentine poem for everyone in his family, plus the cat, the dog, his teddy bear, and the bird that sings outside his window.  Each of the poems (there are ten of them) has the same format:  they all begin with "If you'll be my valentine" and go on to say, in four short lines, what the little boy will do with or for the recipient in return.  The one he writes to his mother is (not surprisingly) my favorite:

If you'll be my valentine
I'll pour our tea at three.
Spicy cookies
and an orange
just for you and me.

Okay, it's a simple poem (a little boy is supposed to have written it, after all).  But I love the specificity of it:  tea is at three (the illustration of the boy and his mother having tea shows the clock in the background); the cookies are spicy.  Also that the boy is doing something with his mom that she would particularly like, although he is certainly enjoying it, too.  This is true of all the poems:  in another, the boy promises to pull his little sister in the wagon so "we can sing and talk."  Milly, a little sister herself, likes that one best.

I had planned to write an acrostic poem for each of the kids and my husband this Valentine's Day.  I still might (even though my husband's name has an X in it, and it's hard to work an x-ray or a xylophone into a Valentine).  Or maybe I'll write these instead:  5 lines, first line "If you'll be my valentine," last four lines ABCA and a promise to do something special together.