So to sum up:
So in other words, you're galactic readiness is the final determinant to your ending, which is the sum total of your war assets multiplied by your readiness rating. All in all the
core best ending is easily achievable in single player, while the
variant best ending can be done in SP but is made easier by MP.
The variant best ending is made different by specific people being alive and the effect of certain choices, but the overall galactic result is the same.