The LBJ series by Robert Caro stands head and shoulders above everything else.

Some other good ones, including a great series about Winston Churchill:

Mornings on Horseback - about Teddy Roosevelt by David McCullough

Truman - David McCullough

The Last Lion - series of books about Churchill. I forget the author.

All of Doris Kearns Goodwin books are excellent, as well. She's covered a lot of presidents.