Used, obviously.
But you should buy from an individual rather than a dealer. You pay a very large markup from a dealer on used cars, and most have a vested interest in keeping the price of used cars as high as possible (because they want to sell you a new car even more than a used car). The dealer also knows better than you when they have a good used car...it's been through their shop, so they know. If it's a "good deal" at a car dealer, then you're almost certainly getting a car that's going to have problems in the future.
Also note that standalone used car dealers are often merely dumping grounds for the cars that new car dealers don't consider worth putting on their lot. So, even though a dealer that only deals in used cars wants to sell you a used car and has no interest in you buying a new car, he also generally only has steady access to the cars that new car dealers take on trade-in but don't want to sell on the lot. (There are also bankruptcy and repossession auctions, auctions of distressed vehicles from out of state that are auctioned as scrap and then re-titled as clean through various loopholes in the law, and a few trade-ins from people buying a used car from that dealer and upgrading. But used car dealers are almost universally selling the absolute worst cars available, and at significant markup.)
Buying from an honest individual, knowing the blue book price, and having a trusted mechanic go over the car before making a decision, is the best way to insure you get a good deal on a good car. Watch for signs that you're dealing with a hobbyist used car salesman. There are many folks that operate small-scale buy-sell operations from their home, having one or two cars for sale at any given time, mostly purchased through the same shady channels as used car dealers.
There are probably exceptions to the always buy used rule. Finding a used sports car that is actually going to last is difficult. Most folks don't know how to care for a performance car, and will drive the hell out of it for a year or two, and then pass it on when it starts getting troublesome. BMWs fall into this category a lot of times. When I bought a 350Z a few years back, I bought new, because the few used ones I could find were already (just a year after they hit the market) pretty abused and had problems.
But I'm not even certain that was a wise choice...if I'd waited another year, I might have found more options to buy used. Regardless, the vast majority of cars don't fall into this category, and the more boring the car, the better deal you can get on it used. My folks bought a two or three year old Camry with 30k miles for very little, like maybe eight grand, a few years back.