I've built a couple trebuchets, as well as an onager.
For those looking for an explanation, a trebuchet is basically an arm with a counterweight on one end and a sling on the other. An onager is a machine powered by twisted ropes that swing the arm around, connected to a sling.
For extreme power, look to onagers.
www.onager.net has a good one, built by I believe Bob Carbo. These machines probably get the most power for their size out of all catapults.
For accuracy and efficiency trebuchets can't be beat; this assuming you take into account the more modern FAT (Falling arm trebuchet) and F2K models.
All this and more on
www.trebuchet.com , which has it's own msg boards.