Personally I wouldn�t choose to live in Wembley although I have relatives nearby, and Harrow is probably too much along the Heathrow flight path for my liking. Ealing is ok, some nice leafy places around there and plenty of shops and cafes near the Broadway. I have lived in Kingston and found it to be very pleasant in the right parts � although the people seemed to have a snooty �we are nearly Richmond� air about them. It had a good selection of pubs and clubs, and of course nearby places to visit (Richmond, Kew Gdns, Hampton Court) with some pleasant Thames River walks. The other consideration is where you would be working as some places have better transport links than others (Kingston is not on the tube network but has good train links nonetheless).
On a safety point, I would personally rank as follows from best to worst:
Kingston
Ealing
Wembley
Harrow
Although of course, everywhere changes when it gets dark, so caution is advised.