Oh boy... Google is big, but limited. As far as advertising goes... I don't think it would make a big difference!
Probably Yahoo, but I doubt Google will go anywhere, it may go through changes in ownership in the future, but as long as there are searches Google will pobably be around. (Which doesnt mean they will always be one of the most popular)
yahoo, although they have the slowest SERP updates msn might be an option too.. easier to get indexed
Yahoo definitely for me, in fact I do alternate between Yahoo and Google to find the best results. And if Google disappear, I am sure there are dozens of new search engines appearing and competing for Google's share of the search engine market, we would be spoilt for choices.