It's probably better to turn your question around: "Which quality regulators would you recommend and why".
I've got a pair of Scubapro Mk25s (used for sidemount; they're a better shape for that) and about 7 sets of Apeks regulators (for twinsets and decompression stages). MK25 regulators work well but are said to be more complex to service.
The Apeks DS4 (Dry Sealed 4 port) are excellent value for money and extremely reliable in cold water (therefore warm water too). These are simple, reliable regulators that are said to be easy to service. They're very good value for money.
The Apeks DST (Dry Sealed Turret - a twistable turret on the top to position the hoses with 4 ports and a possibility for a 5th port option) is a particular favourite for decompression cylinders and for use as a single cylinder first stage. Good value for money, but slightly more expensive than the DS4. They're also said to be easy to service.
I also have a pair of Apeks Tec3 first stages. These are just like DS4 regulators, but are optimised (a left-hand one and a right hand one) for twinset use. I got mine second hand and have done hundreds of dives. They're said to be easy to service.
For second stages, I cannot be bothered with all the expensive ones. I like the simple Apeks ATX40. It's reliable, works very well in cold (and warm) water and very good value for money. I also use those on my Mk25s.
I dive quite deep and completely rely on those regulators. I have no qualms in recommending them.
As a technical diver, I only use DIN connectors - yoke is a joke as it were. All regulators are available in both versions, you can swap them over in seconds (OK, a minute).
I don't really care about the standards. They just work. And they're CE + EN250.
Apeks do a range of shiny expensive regulators. Oooh. They're no better than the two I've mentioned, just three or four times the price! Don't get suckered in; it's all marketing fluff.
Warning: if buying second hand, do get them serviced. Whilst a regulator looks good on the outside, it can be awful on the inside. When they're serviced, they'll come back with a certificate and (certainly in my experience) a bag of the old O-rings, filters, etc. from the service kit.