How do I pick the best fillet mignon at the supermarket?
Marbling
From my experience working in butcher shops, you want to first look for good marbling (unless you are on a low fat diet). To illustrate:

I prefer personally anywhere between No. 5 and No. 8. No. 9 and up I find a bit excessive, though some people like it that way. I have had some below No. 5 and they were still good, though.
Dry-aging
From Wikipedia article:

The process changes beef by two means. Firstly, moisture is evaporated from the muscle. This creates a greater concentration of beef flavor and taste. Secondly, the beef’s natural enzymes break down the connective tissue in the muscle, which leads to more tender beef.

Some higher-end butcher shops will dry-age their own beef (at considerable expense) but if you don't have a shop nearby that does that, you can also dry-age it yourself.
Other things
I would recommend not to marinate fillet mignon and season it only lightly. If you plan on wrapping it in bacon, as many do, if you wrap it a day or even a few hours ahead of time the meat will absorb more of the smokiness of the bacon.