Bioware explains how to get the “Perfect Ending” in Mass Effect 3

If you’ve played or been playing Mass Effect 3 you’ll notice throughout your time with the game that the Galactic Readiness meter stays at 50% in the single player portion, if you fail to play multiplayer before you reach the finale.  This has lead to some confusion by the Mass Effect community, wondering whether you needed to play the multiplayer to get “The Perfect Ending”.

Jarret Lee of Bioware took to the Bioware forums to clarify “The Perfect Ending” and the Galactic Readiness tool in the single player portion of the game.

“I’ve seen some posts where there is a bit of confusion about the ability to get the “perfect ending” by only playing single-player, because if you don’t play multiplayer (or the iOS game, Infiltrator) your Galactic Readiness stays at 50%,” said Lee.

You do NOT have to play multiplayer to get the best single-player endings.
Here’s a bit of clarification:

Your ending(s) are determined by your “Effective Military Strength” (let’s call it EMS for now) bar. Focus on that bar – that is your indicator of how well you will do in the end-game.
You can maximize your EMS just by collecting War Assets in the single-player game. There is a certain threshhold of these you would need to exceed (I can’t get too specific) but I can tell you there are MORE than the required amount that can be gathered in the single-player campaign.
“Galactic Readiness” is a modifier you can improve by playing multiplayer. That is to say, if you play a lot of multiplayer, you will need less War Assets from single-player to fill up your EMS bar (ie it will balance out the requirements to account for you playing in both modes).  Single-player game play does not impact that bar.

EMS = success. EMS can be maximized via collecting war assets alone, even if your Galactic Readiness is 50%.