I think one I would want would be the foghorn, since it seems that some people don't work together the foghorn let's off a screech as the specail ability that gets the ai close by to follow the foghorn. J know the name sounds stupid but that could be changed.
For groups I think the beserker would work, they can use the special to to enter a 'rage' that the charactor looses control and spins and attacks, while in it's rage it's health doubles however it only lasts about 10 secounds.