The “s”s and the “z”s can be as diverse and interesting as the vowels, but I haven’t often had the chance to discover the minute differences between the different variants. The languages around me aren’t as rich in sibilants as they are in vowels and I haven’t had the practical need to differentiate between the allophones. Here is a theory of the “s”s I’ve heard and believe I can differentiate:

Retro Bulgarian (Todor Zhivkov, Baba Vanche): Laminal, Dento-alveolar. It’s hissy, but it’s not a clear perfect sound, because of the teeth, sometimes you can see their tongues stick out. Very front, similar to French and some Germans (“theory” might sound like “seory”)

Modern Bulgarian (me, Lyubi, who else?): Laminal, alveolar. A hushed version of the Retro Bulgarian. Trying to prononunce the Retro Bulgarian makes me feel like I am mocking an old person. The exact position on the hard palate varies depending on emphasis and co-articulation but it’s often where the “sh” sound is, but the groove of the tongue is narrower, so to make a distinction.

USA English: Of course it varies a lot, but there is a very englishy “s” that I believe is done with the tip of the tongue just behind the teeth. It’s the sound you make if you try very hard to imitate the hiss of a snake. The other “s”s here are not so snake-like in comparison. Same for the “z”s. The English “z”s are much closer to the buzzing of the bees. The english “s”s and “b”s I am describing are what I call “perfect” because they don’t remind you of any particular body anatomy when you hear the sound. In comparison, hearing a French “s” you think “Tongue is close to the teeth!”, but an english “s” could be more easily confused for a non-human sound, because it’s so clear and doesn’t remind you of teeth, tongue or saliva. French is like Retro Bulgarian but there’s more “th” in there. That’s also probably why they confuse the English “th” with a “s” instead of “t” like Bulgarians do.

German varies from French to English

What about Russian?