Dried oak leaves are good as are almond leaves. It depends on your location. I have been warned away from walnut leaves since walnuts contain chemicals that prevent other plants from competing with their roots by killing the other plant in a sort of chemical warfare going on at the roots level.
I'd be wary about using anything else than oak and almond too. Many other leaves decompose too fast and some may be toxic to aquarium life (for example, conifer needles probably won't decompose fast, but the resin is likely to be irritating or harmful to fish).
If you decide to experiment (at your own risk ), use leaves that are sturdy and "leathery" like oak leaves, dry them and boil them before putting them in the tank. This should minimize the risk of harmful chemicals making it to the aquarium. To keep them from crumpling up as they dry, put them between towels or some such under a light weight to dry.
To those for whom leaf litter in an aquarium is a new idea, be aware that they'll lower the pH of the water somewhat and can tint the water slightly brownish, much like bogwood does. So if this isn't what you want, don't try it.