Play Shadow of Chernobyl with the Complete mod; unlike some series in which the later games are clear improvements over the first games, SoC is just as good as Call of Pripyat, which is the other great game of the series. I haven't played Clear Sky so I couldn't tell you much about that, but I hear it's sort of the ugly duckling of the series.
Next, play Call of Pripyat, and if you liked the difficulty of SoC with the Complete mod, then you can get another Complete mod made for CoP too. If you felt it wasn't hardcore enough, try the Misery mod, but be warned it lives up to its name. I wasn't a huge fan of Misery mostly because of the really low weight capacity (it may be unrealistic, but I like carrying a bunch of stuff around). This is one of the reasons that playing SoC (the other reason being chronological order; CoP has a major character from SoC); you can gauge the difficulty you want for CoP.
I think you'll probably notice that most people recommend the Complete mods and at least a few people recommend the Misery mod, BUT there are lots of other great major overhaul mods for both games. I'd recommend going to modDB and checking out the changes made by each mod before making your decision. I like Complete because it's sort of a conservative overhaul; it adds some new content, makes everything a little harder, fixes some badly implemented gameplay features, but doesn't make any truly radical changes, so you're still playing the game as the developers intended it, only better in every way. But you might feel more inclined to try a different mod once you read about it. Remember, they're total overhaul mods, so you can only pick one mod per game.
Next, play Call of Pripyat, and if you liked the difficulty of SoC with the Complete mod, then you can get another Complete mod made for CoP too. If you felt it wasn't hardcore enough, try the Misery mod, but be warned it lives up to its name. I wasn't a huge fan of Misery mostly because of the really low weight capacity (it may be unrealistic, but I like carrying a bunch of stuff around). This is one of the reasons that playing SoC (the other reason being chronological order; CoP has a major character from SoC); you can gauge the difficulty you want for CoP.
I think you'll probably notice that most people recommend the Complete mods and at least a few people recommend the Misery mod, BUT there are lots of other great major overhaul mods for both games. I'd recommend going to modDB and checking out the changes made by each mod before making your decision. I like Complete because it's sort of a conservative overhaul; it adds some new content, makes everything a little harder, fixes some badly implemented gameplay features, but doesn't make any truly radical changes, so you're still playing the game as the developers intended it, only better in every way. But you might feel more inclined to try a different mod once you read about it. Remember, they're total overhaul mods, so you can only pick one mod per game.
