Yahho for sure, but they have just lost there way. It may also happen with google in the future, unless Google continue to come up with innovative solutions
In my opinion the top original search engines 12 years ago (around there can't remember) were excite placement, northern light, alta vista, lycos... Those are the old popular ones. Now its google, yahoo, live aol and go
The main issue is not who used to be the most popular search engine but which search engine will BE the best in the future...
It was yahoo who is dominating the search engine that time. Anyway a lot of people do choose yahoo over google because they have really great search results but after adsense people tends to love google already.
It was altavista.com in the late 90's it was very popular along with excite.com and for you europeans it was Lycos.com