WLP-001 though I do feel like I should broaden my horizons. I also have lots of strains in mind to try just haven't gotten around to it.
As for the why, it's clean and ferments dry that's what I have been looking for lately so it works. Also the yeast cake seems to be more dense and harder to disturb than WY1056, that helps if you're bottling like me.
Safale US-05 --- My favorite beers are American ambers and American pales and this yeast works well for both. It can be used with a number of other beers as well. Neutral flavor profile that lets the hops and malts shine. And I like the fact that it is a dry yeast. It is economical and I can buy enough at one time so I always have a good supply in the fridge.
Edit: I also keep a couple of packets of Munton's ale yeast on hand for the occasional Irish red ale. This old stand-by yeast starts fast, finishes clean and still makes an outstanding red ale.
My favorite is Wyeast 1084, the Irish Ale. I've used it in everything from big bad oatmeal stouts, to Irish reds, all the way down to milds and ESBs. Gives me a nice malty balance, ferments fairly clean, and brew's wonderfully in my cold-ish apartment.
1272 for APA/IPA's
1968 for ESB's or English brews
3711 for Saisons
1214 or 3787 for Belgians.
A couple of new dry yeasts are beginning to grow on me. I did a couple of batches using BRY-97 and they turned out good. I just bottled a Saison using Belle Saison. It took it all the way down to 1.002 and it tasted very good on Bottling day. It seemed to be a little like 3711 but with a little more spice. In a couple of weeks after it is carbed up I will really get a good chance to see the final taste.