Get something that you can get serviced locally. I am partial to Apeks/Aqualung and feel I have purchased the best regs for me and I can get them serviced at my local dive shop. I recommend a balanced, environmentally sealed Apeks/ Aqualung.
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...but once you take it to depth the performance changes, and since my thread I made on it I've dove 2 more in the same conditions (to any of you who may have been involved in that discussion) of the same year and model make, and I got the same result: less performance...
Often it depends on the temperature of the water you're diving (some regs work the best in warm water). BUT mostly it depends on what regulator is the most comfortable for you. What did you dive in training? Ask your LDS what they'd recommend (they'll try to sell you something, but try to filter that stuff out). Plus your LDS probably stocks regs that are made for your environment.
I dive an Aqualung Titan LX, which worked well in 5 degree Celsius waters up to 28 degree Celsius water (41 and 82 degrees Fahrenheit respectively).
Just remember that this is one of the most important things keeping you alive underwater, and $300 isn't that much for scuba . And if you buy used, you're going to have to get your regs serviced anyway (probably $50).
There are multiple brands of regs, with product lines that include a) low end, b) low end / high end, and c) high end models, as Herman pointed out. Across 'major' brands, it is hard to say that one reg is better than another, when comparing the equivalent models.I . . . need some reccomendations for a regulator. I'd prefer to buy a good used one and have it checked out but don't want to spend 300+.