Bulgar is preferred over barley by miles. But I like to make a provencal style tabbouleh using couscous. Be sure to use lots of mint and parsley as it should come out really green.

Without questions bulgur. #1 grind is best

I used slightly undercooked couscous the other day and it tasted fine

Bulgar...

Generally bulgar but I recently used a fine grind barley from kalustyan's that had a texture like couscous or bulgar that might be an interesting swap

Bulgur is traditional, and there are several grades of bulgur I use the whole grain health food bulgur, but I believe 'fine' bulgur is more common. If you're near a Middle Eastern market, you'd have these choices.
Some people use the tabbouli ingredients with a different grain. I like to switch in quinoa, preferably brown quinoa.

Bulgur!