Clear Sky was my least favorite, but it's still totally worth playing.and really beautiful as well. It's what I did.I would recommend Shadow of Chernobyl after that, and honestly I have a hard time choosing between SoC and CoP as my favorite in the series. Listen to HEXIT on this as well.go ahead and play CoP first if you want.
The STALKER series of games is among my favorite in all of life. which ever version the complete mod is available for it here just follow the links. its the 1 game series bethista have gotten rite lately. I would recomend you get both as you will not be disappointed in either. New vagas is a good game also and worth the money just as much as stalker is. there may be privatley hosted games but i havent tried to play latley it was about 3 years back when i last played the multiplayer and yeah it is basic tdm.
Multiplayer nolonger works as the servers were shut down. also the mod helps fix a lot of bugs that were never patched so yeah they are essential and dont change the game story.
if you dont use the mods there is a huge difference between the 3rd and first game. the mod adds dx10 shadows and sunbeams as well as bump textures to the older games so they look almost as good as the last. get the complete mod for all 3 and you wont loose any gfx quality as you play. You should play them in reverse order as then you can enjoy the story as the 3rd 1 is the start of the saga.