Which is the most important part of the book? For me, it's the end. If the end is terrible, I tend to forget how much I enjoyed everything that came before it.
How much the beginning matters depends on why I picked up the book. If it's just something that looked cool on the bookstore or library shelves, I may return without reading if I can't get into it. But usually I read stuff that someone recommended - be it a friend, family, or a fellow blogger. Therefore I'll give the book's slow beginning the benefit of a doubt. (One I never got into? An ARC missing the first 29 pages. I couldn't figure out what was going on and decided it wasn't worth trying even though it sounded awesome.)
As for the middle, it's only important if I didn't like the beginning. I have too much to read to waste time on books I don't care about. If I'm not grabbed by page 100 or so, I'll give up.
I try to pay attention to the whole, but I can't say one kind of pacing is best. A story needs to be paced the best way to tell the story. Some parts are necessarily slower, or less exciting, or more expository, and so forth. But when I finish a book and deem it a good or bad read, the end colors my perception completely. It's freshest in my mind - especially if I've been let down.
What about ya'll?