Mentha requienii I think is the one, known as corsican mint. It has very very small leaves which almost look like weeds but has the most fantastic sharp scent. All other mints are fairly fragrant but not in the same league, as with herbs like oregano, fennel etc.

Aside from the common herb plants you can get at most garden centers you should also look at:

Cuban oregano (Plectranthus amboinicus), and other plectranthus species
Patchouli (pogostemon patchouli)
false cardamom (elletaria cardamom
Australian mint (Prostanthera species)

and a whole bunch of others -- look for a book on fragrant plants or a book of herbs at the local library or bookstore.

Scented geraniums probably have the widest variety of scented leaves of any genus around.

Jim

I have a row of lavender along my path and even when it's not in bloom if you brush against the foliage you still get that lavender fragrance. Using creeping thyme on a path between stones or where there is not too much traffic, when you step on it the fragrance wafts into the air. Also sage. I have ornamental russian sage and if you like that kind of smell it is very fragrant.