Ground spices are convenient, but for the absolute freshest flavors, you might explore grinding your own whole spices yourself using a mortar and pestle, a coffee grinder, or a microplane grater. To amp things up even more, toast the spices briefly before grinding.
Ground cinnamon seems to top the list of the one ground spice every baker has on hand. What else you stock depends of what you like to make. For example you might like ground cloves, allspice, and ginger to go along with the cinnamon.