It's actually quite a difficult question to answer. Could be why people are avoiding it. Simplifying things horribly, any reg that fits on the cylinder and isn't going to fall apart is fine, however you get options with gauges and staged regulators. Most of the regs we use in fishkeeping are actually single stage regs, but many are sold as dual stage. Branded regs tend to be very expensive by comparison, running into the hundreds of pounds quite quickly so we get a lot of Chinese manufactured regs, often sold on ebay, and generally they're reasonable pieces of kit.
Basically your decision is based around gauges, how many and showing what. A pressure gauge will give you some warning as to when the tank is going to run out, but it's not a lot of warning, when the pressure drops it means that the tank is nearly empty. Otherwise gauges can show the pressure within the reg itself, and/or the flow rate of gas. Most regs are adjustable, but some come with a fixed pressure (worth remembering that some diffusers need 3 bar of pressure).