I've beaten all three!
SoC is a great baseline for the series with probably some of the best singular moments, but the overall experience is still really rough. Crazy difficulty spike right at the end.
CS I remember sorta as a fever dream. Faction wars was fun, even if it was dumb you could never actually win. I also remember that game also having an absurd difficulty spike in its final act. Just how many grenade icons can show up on the UI at the same time?
CoP is the one I've played and enjoyed the most by far. I think the game just lends itself better to larger maps, and CoP's were just the right size. The story was pretty neat, too.
These days I find the vanilla games to be pretty dry. Back in the day the Complete mods were good for quality of life, but you'll definitely be laughed at for playing those now. I remember a bunch of smaller one-offs, and those were cool because they had custom maps, but my favorite is definitely the Misery mod for CoP. It's got that right balance of item autism, slow progression, and meaningful upgrades without devolving into something that isn't Stalker. Supposedly Gamma is the new hot shit. I've played through Anomaly, and didn't find it too offensive, but Gamma seems really divisive for some reason. Zoomer this. Gun porn that. Blah blah Tarkov. I don't see how anyone over 30 has the patience for that kind of conversation.
No clue about Stalker 2. I expect it to have released in a disasterous state and will only be playable next year. It makes sense that they don't have A-Life if they're using UE5 instead of X-Ray, but I would hope they could remake the system. The game would be sorely lacking that verisimilitude without a reactive world like that; basically just Chernobylite at that point.