Oh goodness, I kinda suck at remembering the differant types of hacks....So bear with me.
I have a friend who rides her arab colt in a hackamore. It's an s hackamore, I think(?) it has the bit shanks (yet obvously no bit) and a soft rope noseband and browband. It's very gentle on him and he responds well, and it's nice and flashy on him for the show.
Now the only time I've ever ridden without a bit is on my little Peruvian Rico and I used a sidepull. I absolutely LOVED it. I also used to ride our stud Jester in a bosal for the "wow" factor at shows.
I've heard some pretty good things about the bitless bridles as well. There is a girl here at the barn who I see ride in one, and the hrose seems very responsive.
I'm afraid this isn't my area of expertise, so I apologize if I typed out a mistake in here. Just figured I'd get my two-cents in :)