I suppose it depends on the specific features you are looking for, but I would say that in general, Google is the most user friendly. Its interface is simple, but it also allows narrowing down of search results intuitively through the left sidebar once the search has begun. You can search for just blogs or just news, or include only sites updated in a certain time frame, just to name a few.
However, Yahoo has some nice features for certain kinds of searches (like those in the entertainment industry). They've recently added a sort of slideshow-like presentation of relevant info along the top of results for some things (try a movie title, for example), which lets users browse the info without even leaving the search results page.