If you like pure FPS with minimal eye sore or NORT too much object, then Cs 1.6 is a must.
Server with mods like De dust 24/7 / CS assultt 24/7 or Deathmatch are where all the fun is .

If you are looking for something fun, there are some zombie mods around with the most popular being Zombie Escape.

Even after 20 years, the game still lit with countless servers all around the word, if you have any trouble finding server during log in, just search for it online :D

Coming back to Cs.16 after so many years feels nostalgic.