Hitman: Blood Money is the best in the Hitman series. Bar none.
The game begins with 47, well, dead. Sorry for the spoiler, but its true. A reporter comes to the mansion of a man in a wheelchair to interview him about 47. The missions are actually the descriptions the man gives the reporter of 47's crimes. The story is great, and the missions are all well thought out and unique in their own particular way. One mission has you assassinate an important man in the Opera, who also turns out to be a pedophile. While another has you kill some important figures in the gun trade inside a semi-S&M club called "Heaven and Hell". The gameplay is definetly the highlight of Hitman: BM. After you complete missions, depending on how well you complete the level, you get money with which you can upgrade your weapons. Sometimes, they can really come in handy. Also, the game is very realistic. Bullet holes show up on the body when you shoot it. Leaving your suit around the crime scene impacts your noteriety level, which may involve having your cover blown and the police coming after you.
The sound is good, but it has its weak spots. 47's voice is done very well, by the same man who has done him previously. The guns sound realistic sometimes, but they can have their flaws. Also, the grunts and dying sounds of your enemies are pretty shallow and poor. They all sound the same, which isn't very good.
The graphics can be described in one word: fantastic. The character models look great and 47 looks great as well. The guns look almost exactly like their real-life counterparts. The buildings and environments look really well.
All in all, Hitman: Blood Money is without a doubt, the best Hitman in the series. The graphics are fantastic, the sound is pretty good, and the gameplay is phenomenal. I would highly recommend it to any fan of Agent 47.