The best Rebels are the ones who are in and out before anyone knows they're there. So, you want a low score.
Rebel score is calculated by how many minutes it took your team to finish. For example, a team finishing in 43 minutes has done better than one completing the game in 61 minutes.
Then, add in any points from solutions used or stun mines hit. For example, if you viewed one 5 point solution and stood on a single stun mine then your final score is 43 + 5 + 10 = 58.