For very steep slopes difficult to water:

Prostrate types of cotoneasters.

Cultivars of scotch broom (not, of course, invasive wild brooms).

Dwarf English ivies (not invasive regular ivy).

Boston ivy.

Algerian ivy.

Akebia.

Thornless caning berry cultivars.

Climbing roses.

Mexican evening primroses & shrub sages & succulent ice plant (if slope is
in fullest sun).

-paghat the ratgirl

--
"Of what are you afraid, my child?" inquired the kindly teacher.
"Oh, sir! The flowers, they are wild," replied the timid creature.
-from Peter Newell's "Wild Flowers"
Visit the Garden of Paghat the Ratgirl: http://www.paghat.com