I'm going to assume you meant CU Zaku II since none of the CS Zakus seem particularly tanky...
Why would the CU Zaku II make a better tank than the CR GM III? They have the exact same skill set and the GM III has 3 more Defense off the bat. If you mean the weapon set, well I'm not really looking at that. I'm just looking at sheer tanking ability.
As for the CR Rick Dom II, I've already got two of them and one has a tank build so I'm glad to know I made a good choice in C-Tanks.
Server: NA (Don't judge me... <_<)
Skill Status: Huge n00b (Don't judge me... <_<)
Favorite Unit: Nu Gundam(Don't judge me... <_<)
Short Term Goal: N/A
Continuing Goal: C-Rank Army
Long Term Goal: Reach OC EX on all Nu Gundams (Nu, HWS and Hi-Nu)