The BEST moment in one of my Games was one I ran about two years ago, The setup was that there was Three groups of peaple playing under me. Each with four players. One I ran on Friday, one on Saturday, and one on Sunday, they all had the same BBEG who was like CR 28 or something, I never figured it out. And I had his General playing against each. Well, at the end of that Loooong campaign everyone in each group was level 20 and all of them just had gotten to the BBEG, so we had a single session for everyone. A HUGE group, but it was only one combat, and ended up being the greatest combat that i've ever ran.
That is the most incredible moment. And the power levels were over 9000....