The slower the juice process is, and the least amount of heat produced is best. You want to avoid causing oxidation (juice can last a day or two rather than hours this way). Heat and oxidation will lessen the nutrient values of the juice drastically and spoil much quicker.
Depending on what kind of vegetable or fruit you are juicing, you have a choice from two main types of juicers. Masticating and centrifugal. Masticating is much better used for leafy greens, berries. Small/thin objects or light weight objects. Centrifugal are much better served for fruits and very watery objects that are dense. Centrifugal however run at RPM of several thousand in order to juice, while masticating are simply a slow pressing juicer. So you can see which one is more beneficial for longer lasting and higher nutrient juice, but with either you will be limited to what you can juice. This might help you decide which one to lean towards. For centrifugal I recommend
Breville (I owned one before and enjoyed it). Centrifugal are much harder to clean and will take longer to maintain. Masticating have very few parts that require cleaning and can be rinsed easy. I'd recommend
Green Star or Omega for a masticating juicer.
There are new juicers now that are two-in-one. Run at a much lower speed, juice both kinds, and operate at a low heat. If you are going to do both kinds of fruits/veggies then I highly suggest it. I'd recommend the Omega Vert 330 or 350
found here
As stated in previous posts. With juicing you will lose the fiber (it can be recovered and used in baking). One thing too, is to be careful with calorie intake.. you can put in several fruit and end up with a 1000+ calorie drink without even blinking an eye.
Hope this helps.