brittle stars are fine.. avoid green as they can get aggressive/mean..
Blacks are neat.. be aware they can get pretty darn big fairly quickly..
I started with a small ~2" across one and it was 24" across in under 2 years..
They really only come out at night though and will usually just stay hidden under rocks,etc..
Oh and they WILL dig out sand so ensure your rocks are stable/sitting on glass or whatever..
Brittle stars are fun to watch and do a hell of a job keeping the tank clean, especially if something dies that you don't know about. Touch a piece of frozen shrimp to the water surface and watch it climb up the glass and grab it from you
More standard looking sea stars might be pretty but they are boring as hell and provide little to no actual value to the tank. Plus they are either hard to keep alive or are not Reef safe.