It didn't take me long to find my answer. I always chose the enzyme. Enzymes are protein molecules made up of amino acids whose purpose is to catalyze reactions between other molecules called substrates. Chemical reactions are occurring thousands of time per millisecond within your body, and if not for these protein catalysts, something so rudimentary as digesting that simple snack would take eons to achieve. In fact, you'd starve to death before the food made it into your gut. Literally. Enzymes make some reactions occur 10,000 times faster than they would without help.
But if you ever play this game, and wonder what the perfect particle would be, I have a little suggestion that will turn heads and make others go wow. In a word, be a neutrino.
Discovered in the 1950s (an achievement which earned the scientists a Nobel Prize) neutrinos are nearly massless, subatomic particles. The universe is awash in them, but just because they are prevalent doesn't mean that they are at all common. Neutrinos were created in tremendous numbers right after the Big Bang and are constantly being churned out in stars. These so-called ghosts of the universe (because they are so small) sail right through galaxies, including our own. They carry no electrical charge, and so are attracted neither to protons nor electrons, so they are not bound by electromagnetic fields.
So even if you find yourself aloof or don't care to play traditional roles in society, even if you sometimes feel hardly even here (I myself fit that description sometimes), know that your purpose on the Earth, in the galaxy, and in the universe and to the 8th dimension and beyond, is essential to the survival of it all. Just by being here, your presence is felt and a benefit achieved. (A friend once used the term "consciousness holder," which I think is pretty apropos.)
Now doesn't that describe you to a T?