A few things that travel well to a potluck:

Dishes that taste good at room temperature, like grain salads, roasted vegetables, or a vinegar-dressed potato salad.
Anything that holds in a slow cooker: chili, beans, meatballs.
Food that's easy to serve with one spoon.

A small card with what's in it helps the friend who can't eat nuts, dairy, or pork.

The best potluck dish is the one that's still good an hour after you get there.