Since everything else has been covered (especially clowns! A pair is great, 3 is a recipe for murder), I'll add my $0.02. Yellow-tail damsels provide a great pop of color and activity at a very small price.
Yes, they are territorial and aggressive to newcomers. But if you add it last, it will be much less territorial. If you do add one and decide you want a new fish down the road and the damsel is aggressive, you can either move some rocks around (particularly their "territory") to throw them off, or pull the damsel out and keep it in your hospital/qt for a few days while the new fish gets acclimated.
I'm in the minority. Most people hate damsels. But they are neat, beautiful fish if you handle them the right way.