it depends
Z2 doesn't have a swivel, isn't enviro sealed, and is a piston. Means potentially less than ideal hose routing, and a need to be extremely diligent when diving in salt water to make sure it is flushed properly. Made in the USA if that means anything to you, even if it is in CA ;-)
Aqualung Titan QC is a gimmick imho, don't bother with it, no need in recreational diving to need a reversed regulator. It is also an unsealed design, so needs to be taken care of when diving in salt water.
Aqualung Core has the ACD which I'm not a fan of because in the eternal words of
@cerich it is an equipment solution to a skills problem. Again, it is an unsealed diaphragm unless you get the Supreme so you have to pay special attention to rinsing.
If you noticed the trend, I don't like unsealed regulators, although all of mine currently are unsealed, they are Poseidons which have a different top portion that makes cleaning much easier, and >99% of my diving is in fresh water where it is less of an issue. If I am diving in the salty stuff, it is almost always followed by diving in fresh water where they get fully cleaned out.
Of those three, Atomic. I wouldn't buy from Aqualung directly, would choose Apeks. My question to you though, is what do you have now, and why aren't you happy with it?