Blue Green Chromis are real pretty, but lots of people have em'.
I have a yellowtail damsel and he has no agressiveness in him whatsoever.
I guess it all depends on which one you happen to get. Some are some aren't.
I had a blue devil damsel but they are EXTREMELY agressive and territorial.
I think if you got a big school of yellowtails that would look nice.
That's what my dad has in his 55 and it looks pretty good.