I've played the three of them and I'll give you my opinion:
Culpa Innata is a mediocre game: it has some great features (mainly the background story, based on a novel by Alev Alatli), some mediocre features (the graphics, the dialogue mode) and some terrible features (some puzzles are idiotic and the characters are nothing more that flat shapes of carton).
Syberia I & II: Depends on what you like. Both games have great stories and are plentiful of characters that, before the very end, you will strongly care about. The first chapter have also one of the most powerful ending I've ever seen. The major flaw of both games is the puzzles design: for one or two logic puzzles, you'll find dozens of mechanical devices to get working, levels to be pulled, robot - but don't call them this way!
- to repair, tiles to be adjusted and so. If you like this kind of puzzles, you'll love the games.
Post Mortem is, in my opinion, the weakest choice. I found it simplistic and banal, quite stereotypical in both story and characters. If you plan also on playing
Still Life, it might be a good idea to play P.M., which was almost a prequel. If you don't, I'll leave this game on the shelf (as I'll do with
Still Life 
)
