The only completely missing chapter is a hit set in the rainforest, in which 47 had to befriend a tribe of panther-worshipping natives by retrieving their lost idol - yes, really, and it was just as silly then - and fight a Scarface rip-off with an odd immunity to bullets. It's not a huge omission in practice though, because Contracts includes updated versions of almost all its levels along with a handful of new ones.
The original Hitman: Codename 47 from 2000 is notable by its absence, and would have been good to have just for historical interest. This triple-pack covers almost his entire early career: Silent Assassin, Contracts and Blood Money. He's the world's greatest assassin travelling the globe to bring a civilised kind of murder for hire to those who deserve it, and somehow world famous despite his greatest talent being getting in and out with nobody knowing he was ever there. His name is Agent 47, and I do believe you know his work.