I've been using Ed Sheeran's Shape of You quite a bit recently as a fast flat road in my classes (around 99-100 rpm). It's perfect for keeping your riders focused on maintaining those upper range rpm's. I like to challenge my riders to maintain the pace and try adding just a bit more resistance each minute.
If you're looking for something a little different, try the Galantis Remix…:
…or even the acoustic version for a cool down/stretch: