Wow, thats a hard one
on the poll I picked the Atomic and the reasoning for that was I own or owned something close to all the others. I own an ATX40 Octo, Scubapro G250 MK/20, G250 MK10, and Poseidon Jetstream / Odin. So I would have to go with something new and that would be the Atomic. I have never used this regulator but it was designed by ex-Scubapro employees
so it must be good! On the other hand, I like my regulators to some kind of adjustable control knob (less my Poseidon) hence, the G250s I own over the S550. If I had to own only one of these regulators it would have to have this adjustability (for colder water) so either the Apeks or Atomic. The Poseidon is a great choice for deep cold water but it tends to breath for you sometimes
maybe I just need to get it detuned a little