I really do not recommend lost alpha or any of the sort that changes the main story of the game. Why? Because it is really important that you complete the story first before doing anything like that. The story has a rich and immersive feel and shouldn't be washed away if you haven't completed it beforehand.
In any case, For mods I recommend, the mods that polish up the game such as bug fixes and quality updates.
and for that case I recommend :-
S.T.A.L.K.E.R.: Shadow of Chernobyl
STALKER Complete
S.T.A.L.K.E.R.: Clear Sky
Clear Sky Complete
S.T.A.L.K.E.R.: Call of Pripyat
Misery
Misery is such a good mod and brings so much to Pripyat. I don't think I've been as scared without misery, the effect, sound, deft and story. It all comes out in misery.