the GM sniper, the 1/144 is not worth the money.
The second, the Zaku I sniper, is quite the nice kit, its one of my favorites, I have two, if you check my www.bakuc.com profile you can see them (pics suck lighting was off)
The third is a new kit... sort of. Its the old Gm{G} kit from the MG line, its not a bad build, but could be better, arm armor doesnt stay together well without being glued in place. (I dont like to glue the armor on if I dont have to) But it has a sniper rifle, is molded in dark green (doesnt matter to us anymore) and has an energy accessory.
Out of all of them, I would get the Zaku I sniper every time.