This is not going to be an exhaustive discussion of regulators. Suffice to say that there are many excellent models on the market from great manufacturers that work really well. However for the beginning diver there are important questions to answer before you decide on a specific make and model.
The first criterion that I would look at is the type of diving you will be doing. If you will be diving in warm tropical water, then you would most likely be selecting a different regulator from the person who will be diving in mostly cold water. In regard to cold water, one should select a regulator that is environmentally sealed. What this means is that very cold water will not (should not) cause a free flow.
The second criterion that I would evaluate is the mass (weight) of the actual second stage. If you are prone to jaw fatigue you would want to select a second stage that is as light as possible. Many of the newer second stages are made from a durable lightweight plastic material that performs very well.
Another factor to consider is whether there are additional ports available on the first stage. Additional ports will allow you add more hoses later on if you need. For example you might want to dive with a drysuit etc.
Alternate air sources (octopus) have to be clearly visible and are normally yellow. Most divers usually select an octopus that is compact and can be used in the up or down position. This will become clear to you when you start practicing donating an air source to your buddy. When you hand the octopus to your buddy, you don't want to waste time making sure that it is in the upright position. It should work in any position. If you will be diving with an integrated air source on your BCD, please see my previous post about BCD’s.
If you are a large framed person or tend to breathe very hard, you might want to determine if the regulator will be able to supply you the amount of air that you need. The higher performing regulators are very hard to "over breathe". This means that even if you do a strenuous and/or deep dive, the regulator will still be able to supply you all air that you need.
With regulators, you typically get what you pay for. The higher priced regulators generally perform to higher standards and allow one to do more advanced types of diving. However, most current regulators from reputable manufacturers perform very well for the recreational diver.
I am not going to make a specific recommendation about a manufacturer or model. I think it is best that you determine what works best for you with the help of your local dive store (LDS). In fact I think it is important that you have some kind of relationship with the store, since you need to have your regulator serviced every year. Additionally, most good stores will allow you to try out various models in the pool if you are not sure of what works best for you. Don't feel you need to make a decision and there is no going back. Make sure that the regulator you select is comfortable, performs beyond the level of diving that you will be doing and that you have it serviced regularly (I know I have said that twice now - but you know how it is with instructors...)