Codename 47 has brilliant atmosphere/music for its day, but level design and game design and controls are dated and I really do not recommend it (maybe watch a quick letsplay instead).
Silent Assassin has some bad levels, but once again, the music, atmosphere…it is worth experiencing. It fixes most of the rough edges of Codename. Just listen to this
Contracts is a half remake of Codename 47 and half new, I recommend it a lot, it is probably the darkest game in the series with some great level design. Last mission is basically a IOI take on Leon.
Blood Money is a masterpiece and best game of the franchise to date. Totally holds up on PC too.
Absolution is…hmm. It is a fun game with great atmosphere, very grungy, almost grindhouse-like, but it is not a Hitman game. It is closer to Splinter Cell in design than Hitman. But still fun for the most part. I do recommend it overall.
Hitman 2016 is return to huge sandbox-like levels, and pretty brilliant game overall. My only issues with it are lack of Jesper Kyd - the music here sounds more Bond-like than Hitman-like, which hurts atmosphere - and very low emphasis on story. I actually liked more storydriven previous games.