Anything that does something autonomously....
A.) Two Stage flights
B.) Airstarts/Clustered motors
Close to these would be really fast acceleration flights...warp 9's and even some of the smaller F flights that have been neck snappers.
Anything larger than a K is really cool, but in this day and age, after you've seen one M you've kind of seen 'em all. Ok I'm kidding. There is nothing blasé about an M.
My favorite/most memorable launch had to be Steve Eves Saturn V at MDRA, and the Red Glare O drag race. The air was still and the smoke columns and trails just hung around on the pad for what seemed like 15 minutes.