Pretty hard to pick just one, but I'd probably go DSOTM...especially when you consider when it was made. There wasn't anything like that already out there when they released it and the recording tools available at the time were pretty primitive by today's standards. After 40+ years it still holds up and has a wide appeal.
There are a lot of good (and some off-the-wall IMO) candidates in this thread, but I think to be the "best ever", it should at least appeal to a wide audience. Pet Sounds, Boston's debut and Abby Road (or a few other Beatles choices) are also very good candidates.