Ask a dozen barbecue experts this question and you'll probably get a dozen different answers. It's a really individual preference based on what you can afford, what you are used to and what you have good success with. The bottom line is, a skilled pitmaster can create a great smoked product with just about anything you give him. There are teams right now who are having great success on the barbecue competition circuit cooking in old rusty trash cans that have been converted into smokers!
Smokers fall into two basic categories; wet smokers and dry smokers. As the name implies, the wet smoker makes use of a water pan which is situated underneath the grate that the meat sits on. The idea behind this is that steam is created which supposedly helps keep the meat moist and juicy. These are typically the upright bullet type smokers made popular by companies like Weber and Brinkman. They are normally fairly small and inexpensive. The drawbacks to them are that if you need to smoke large quantities of meat you'll need several of these smokers since they don't hold much. Also, if you need to cook at a higher temperature, they often have trouble maintaining a temperature much above 225 degrees. Usually when I see a team cooking with a bullet smoker, they'll be using several.
The dry smoker is the type my team uses and the type I use at home as well. They are typically horizontal or upright in shape and have the fire box off to the side. This is what is commonly referred to as an "off-set" smoker. The heat from the fire box moves into the main chamber where the meat is sitting. Although it is obviously extremely hot inside the firebox, the heat inside the main chamber will stay very comfortably at anywhere from 180-275 degrees, depending on how much fuel you have in the firebox and how you control the air vents. Popular off-set smokers are made by The Good One, Traeger, Weber, Blue Ridge & Tucker. Not all smokers use off-set fireboxes. Some, like Hasty-Bake, have the fire box underneath the cooking grates but allow you to raise and lower the coals to reach the desired temperature. And don't let anyone tell you that a Weber Kettle charcoal grill can't be converted into an excellent smoker. Simply arrange the fire to one side of the pan and put the meat on the other side, or put a water pan in the middle and arrange the coals around the outside of the water pan.
Whichever type of smoker you choose, which brand you settle on, and price you end up paying, make sure your smoker has a temperature gauge that actually indicates degrees Fahrenheit. The gauges that indicate cool, medium and hot are worthless and a smoker with no gauge is even less predictable. Remember, the key to tender, juicy smoked barbecue is low and slow, with a consistent temperature of somewhere between 200-250 degrees. I like to go with about 225.