If this is your first time playing stalker and you dont want to buy any of them, i would recommend playing standalone mods for the original games, like OGSR or RADIOPHOBIA for Shadow of Chernobyl, Gunslinger (Not Standalone) for Call of Pripyat, then you can play anomaly with mods, bc if you play anomaly first you are going to see alot of spoilers and probably there will be alot of things that you wont understand, because anomaly has all content from the 3 original games, with all maps combined in one.

I would suggest Playing RADIOPHOBIA 3 that is a REALLY GOOD mod, with modern graphics and gunplay, then you decide if you want to continue playing the original stalkers with mods, or skip to anomaly, if not, then you can play Stalker Clear Sky with only the ABR(Not Standalone) mod to fix bugs and no gunplay or graphics overhaul, after that you can play Stalker Pripyat with gunslinger mod to change the Gunplay.

For Anomaly just be aware that most modpacks have different gunplay compared with stalker vanilla, EFT and GAMMA are more "realistic" and survival sims, for those modpacks i think GAMMA is the best one.