For many years it was on the left, then over the course of a couple of OS X releases a) the vertical Dock became uglier, and b) the horizontal Dock became prettier.
I'd much rather have the user-configurable Apple Menu back, though...
On the first week I used on the left. Then I tought: "ok, let's try on the bottom like everyone". So, my mac has 3 weeks and I use at the bottom, very tiny, and not hidden.