This just came to me for episode 3x22 (the finale). If Bonnie didn’t save Klaus and just let him die which would have triggered the deaths of Stefan, Damon, Caroline, Katherine and Tyler. So when Rebekah goes and kills Elena the way she did, Elena would have died as well, as it was Damon’s blood running through her veins. So that’s 6 main (and favorite) characters she saved in a single act.