i like the door-frame pull up bars because they are unobtrusive, portable, and double as push up bars if you want. but, what you get depends on your goals. do you want to be able to rock the muscle up? you'll need to get something tall and free standing. if you want something outside, a power tower works pretty well.
bargain-wise, the door frame pull up bars are the best. i think iron gym makes one that usually runs $20-30.