You really need to watch the mouthie mouth mr. "f'in re-re" from the other thread. Harridans fly 24" according to the imperial armor apocalypse rules. Now, it could be that they are not flyers, but it's rather irrelevant. They still remain on the board, shoot a HUGE number of high strength shots that can penetrate armor, have enough gargantuan creature attacks to be useful in assault (and if you're smart, due to their nigh invulnerability in melee you will chrage them into assaults just to stretch their move), etc.
The other gargs, in my book, are either too expensive (Hierophant) or too fragile (Hierodules are as easy to kill as a fully upgraded carnifex, but are much easier to see / harder to hide).
Trygons are perty cool tho.