I pretty much exclusively use Google. The only time I'd switch is if I know the site I'm looking for, can't remember the name, and can't find it on the big G. Then I'd go Yahoo, then MSN, then Ask (that's the rebrand of AskJeeves). Google is in my mind undoubtedly the best search engine, and although you're bound to find a few disgruntled webmasters moaning about "rubbish results", these are generally just people who don't like the fact that they are crap at SEO!
Proof that Google is the best is entirely with its market share. They entered the market and gained their massive share by being the best, simple as. They showed that the best engine will become the most popular, and that newcomers can overturn the old heads - I mean where's AltaVista and others now?! There's no reason why another SE couldn't turn this around and displace Google, the reason none of them do is that they simply aren't as good.
Having said that, when I'm doing SEO, although my efforts are primarily focussed towards Google, I will still check ranks in MSN and Yahoo (probably not in Ask), and if I can do things that will make them happy, without affecting Google then even better. I've found it pretty easy to get ranked in MSN, even for reasonably competitive keywords. They seem to place a lot less relevance on links, and more on content - perhaps why their results aren't up to much!
In case you wanted the short answer rather than an essay, then Google.
