Might as well jump on the band wagon....
Shield Dust
I feel the majority of the other abilities really only come in to their own when paired with a complimentary second ability. Which is banking on too much. As stand along abilities, 'Shield Dust' does seem a solid choice.
As for other possibilities:
'Trace' - I'd really feel this would be most effective on, and should be reserved for, a Steel type (preferable as Steel/Grass!! heheheh...)
'Mold Breaker' - Really would be best on a Ground type.