Which of these languages...

is the most worth learning, in your opinion, and why?

Russian, Turkish, Urdu, Punjabi, Korean, Hindi, Arabic, Chinese, Bengali.

Additionally, which countries that speak these languages are pleasant and which are shitty?

Well I for sure can tell you what not to pick and that would be Bengali,  maybe punjabi and Urdu also. The general region that speak those languages are really under developed and somewhat third world. I'm speaking from experince here, native Bengali speaker and lived in Bangladesh for a few years as a kid and have gone back a few times. It's just a really bad area. Horrible smells , roads, buildings, and most things in general. No one wants to stay they all are trying to get out of the country by studying like crazy. Also its really fucking hot and humid.
 
tl;dr dont choose a south asian language