Mandarin, but it’s infinitely more complicated than Russian.
What about simplified Chinese? Still very complicated.
If you study any of the above, make sure emphasis is on TALKING. 80% minimum of instruction needs to be you actually TALKING.
The grammar – formula approach in these takes forever and slows down the capability to talk – too many thought processes.
Learn the way it comes naturally to humans – by talking. That’s how kids learn and it’s a lot faster. Grammar details and formulas are for AFTER you can talk. First learn the what, later the why.