I cannot readily compare Half-Blood Prince to 1 or 2, because those movies are set in their own style.
This all being said, HP:HBP is my favorite compared to 3 and further because of the masterful use of comic relief considering the dark plot, as well as the development of the relationships between the characters. 3,4 and 5 are great movies, but their tones are generally so dark that they provide a stark contrast to the first two films. This movie brings light back to an ever-darkening series with comic relief and as the children find romantic interests between themselves and others at Hogwarts.
Having already seen this movie twice, I would very well recommend it if you have seen the previous ones.