I had a choice, which assembly for cs 1.6 is best to choose so that there is no lag and there are no errors in the game? I do not understand them very well, I saw some forum, they write on it that there are many assemblies, but many of them pull viruses with them.

What build are you interested in? There are a lot of them, from anime to World War II. Personally, I like to play the original English version, my friends recommended it to me and found it here (all-cs) there are no lags, viruses, etc., in general, everything you said, I think that this will probably be the best option for you. Only here you most likely won't be able to play online, I can't do it.