Well, my fave set isn't exactly a set: it is a game, Sharks and Goldfish. You might call it something else, but the game is pretty universally known.
We generally spend the last half hour ever Saturday morning practice doing this, from the littles through the bigs. Littles use their short fins, bigs don't. It covers a variety of what I consider essential for our team:
1) Teamwork;
2) Multi-age;
3) Hypoxic;
4) Sprint-assisted;
5) Coordination;
6) Agility;
7) Body awareness;
8) Situational awareness;
9) Tactics and strategy;
10) Entertaining (I do not use the word "fun" in our programming).
Now, it takes a lot of energy to race back and forth underwater across a pool. Strange thing is though, that even after a really intense morning practice for the upper end of the club, they always have enough left over to play this and give it their all. It might not be as "useful" as some sets, but it fits the bill.