My opinions:
I own and dive SP's MK5/10/15/20/25s Balanced Adjustables,G250/500/600s, Atomic B1s/Z1s, Apeks TX100s,DS4s and Poseidon Odins/Jetstream/Cyklons. All have their ups and downs, their quirks, advantages and disadvantages. They're all great regs, and to tell you the truth, they don't differ much in performance.
For general diving, all would be great with Atomics on top of the list. Silky smooth breathing!
For deep/extreme dives, I use the Apeks and Poseidons. Though I dont do ice or cold water dives (I live in the tropics), having a diaprahm Ist stage is an advantage especially when I dive in murky, contaminated and even polluted waters. (I dove inside a WWII Japanese wreck before with blobs of oil floating on the ceiling. Once it got hit by the bubbles we exhaled, it made droplets the size of basketballs. Unsealed piston regs are a no-no here!)
For servicability, the piston regs from SP and Atomic are easier to service since they have less parts. The Apeks are also easy to service. The Poseidons are another matter, though not hard to service. The 1st stages have so many parts, though finicky in tuning, once you get it right, everything will work out fine.
For cost of maintenance, SP's, Atomics and Apeks are the same with Poseidons a tad higher.
Availability of parts I would go for SP's(except for the really old ones). They, like Aqualung, are like rabbits around here!
Next would be the Poseidons and Apeks. The Atomics, sad to say, are last on the list. They're so expensive that my LDS had to market a different brand!
Costwise the SPs and Apeks are at par with execeptions to the SP titaniums. Poseidons are a Tad higher to outrageously expensive for their outrageously so simple new reg! Very low parts count!
Personally, if you really have to judge a piece of equipment, you really have to own one or try one for yourself. Do regular dives with it...not just once or twice just to test it. I gather reviews from forums, magazines and from other people. From there I narrow it down to a selection and use them with my dives if available. Then I decide which is best and what works for me.