Um, anything during Happy Hour for sure!
But my top faves are either an orange creamslush, a green apple slushie, or a diet coke :D
There are no Sonic locations where I currently live, but when I lived in Florida, I tried the cherry limeade one time. It was excellent. I've tried cherry limeades from other places, but they don't even come close to Sonic.
cherry limeade from the hours 2-4