I have a 4nine, copper panzer and a copper kulturo.
From my experience, the 4nine proved to be the easiest to use. The locking rings on my panzer and kulturo has seized for god knows how many times. As the locking rings are reverse threaded, every time you put your battery in after charging, you will twist anti-clockwise to tighten the part of the mod with the locking ring to the rest of the mod (at least thats how I do it), and after tightening it, you have to twist your locking ring again in the anti-clockwise direction to lock it and that tightens the mod part even more (idk if this even makes sense anymore). This results in the switch (firing button) part of the mod to be overtightened and stuck to the mod, and the next time you want to take your battery out again, you have to twist clockwise with a lot of force in order to unthread the switch from the mod, and your locking ring gets stuck to the switch because your used too much force.
TLDR: get the 4nine/SMPL/praxis mod which does not have a locking ring but won't fire when placed on a flat surface