You cannot get the best ending without playing multiplayer. You need 4000+ EMS [effective military strength] to get the best ending, and that's only one of three choices.

By playing the campaign alone, you cannot get the 8000 war asset points [EMS is the war assets*galactic readiness, which always stays at 50 with no multiplayer] to get the 4000+.

To tell you how to get the best ending it'd require a small amount of spoilers so if you want to read the small [and I promise small] spoilers click on my comments.