I never played a Stalker (I read Strugatzgis Piknik na obotschinje though) or Metro game but after seeing Metro Exodus at E3 I want to get into these ukrainian games.

Which Stalker is the best to start with or the best if I played only one of the three games?

Youll get quite a cross section of answers, but my advice is this:

shadow of chernobyl, with the "SHOC starter 2017" mod. itll provide you a great bug free vanilla play of shoc, with some great additions, but without any of the overwhelming stuff alot of the major supermods add. (thats for later)

avoid clear sky.

secondly, call of pripyat, with "i work alone" 1.2, an older mod, but as above, gives you a pretty awesome "vanilla" although, you dont need to mod cop for it to be awesome and bug free (ish) unlike SHOC, but it will help, especially with weapon graphics, and horizon distance.

these are just for starters, if you get addicted (you will,) then the list becomes alot more extensive.

im sure others will offer you opinions, however, i would avoid many of the older mods, such as the completes, amk, oblivion lost, etc etc, theyre very sub par nowadays, though very good for their time.

call of chernobyl will get mentioned, but thats mainly a heavy freeplay mod and although awesome, shouldnt be recommended for someone new to the series, same detail for misery. however, as youre a fan of roadside picnic, i also recommend, the roadside picnic addon, with call of chernobyl. thats my favourite mod, and i suggest you install that "ASWELL" as the normal game,

either way, i believe you will enjoy your time with the games, they are my most loved pc games ever.

But would Clear Sky not be a good way to begin with? I read that it reused a lot of stuff from the first title in an improved way.

if you want to, by all means, however, just because chronologically, its "first" doesnt mean you should play it as such, and also, its not very good, honestly, i mean your mileage may vary but it isnt.

its your call in the end, this is just advice.

I thought Clear Sky was pretty good. I've logged hundreds of hours into all three and I think all three are great in their own way. That being said, I agree that they should be played in order of release and not chronological story order. Shadow of Chernobyl will always be my favorite though.

Mh, I think if you start with Clear Sky, playing SHOC afterwards might feel like a step backwards. Some features of CS do not exist in SHOC, e.g. nobody in the zone knows how to repair anything (though you can mod that in), weapons and armor cannot be upgraded, etc. SHOC also has worse graphics, though there are now many ways to improve them.
CS also has the potentialy confusing faction war system, which you will have to deal with right from the beginning.

I didn't finish CS because I had to re-install my entire computer (unrelated to the game) when I was close to the end. Back then I played with CS Complete, which still had major bugs (such as level change portals changing locations and popping up close to traders), though if I check modDB now, there are many mods for CS available.

Long story short, I would start with Shadow of Chernobyl with the above mentioned Starter Pack (be sure to check the ZRP configuration tool, it will give you a good idea about what settings are in which file, but allow you to change the important ones via a GUI).

Maybe get Clear Sky when it's on sale. I'd certainly get Call of Pripyat for the amazing mods it has.

You should start with Shadow of Chernobyl.

I first played shadow of chernobyl,then clear sky and then call of pripyat. This is the way the game were released anyway.
I say you should try the same.