You could pick any of the Silver Line (most crowded) and the various Logan buses (11, 66, 88 as bpe mentioned). The buses are the fastest and easiest way. If you've got time, I would walk from the Terminal A side, basically cutting right through the parking lots into Terminal E. I'm not a fan of going through Terminal C, but it is possible.
Edit: The buses (including Silver Line) will be easy to find, right outside the terminal on the arrivals level.