below is based on my experience only, read it with caution :) btw, the sampling size is 100,000+, over recent decades.
personally, i think that people from taipei have the highest marks in comparison.
taipei - 100
pronunciation is crisp, clear, amongst all generations. people are civilised, polite and tolerance to mistakes.
rest of taiwan - 97
with accent, very slightly, others are same as above.
i can guess the origins from hearing their mandarin.
native of peking - 95
pronunciation is clear, just the para-language aspects are inferior to taiwanese.
residents of peking have a mixed result, from average to good.
foreigners learning mandarin in peking; errr, they're too feminine. i suspect that the teaching is deliberate, trying to "internalise" a coordinate attitude into the language for foreigners.
shanghai - 90
with accent, still easy to comprehend, again, the para-language aspects are, aiii, you guess it :(
again, foreigners learning mandarin in shanghai, too feminine in expression.
people from hunan, sichuan and fujian, their mandarin are quite difficult to understand.
the rest, the young generation (20 - 40) are good, older one are poor to average.
if you can learn mandarin from a taiwanese colleague, don't miss the chance; you made good karma in the past :)
btw, may i ask, who choose the chinese name for you? it's a very good one for foreigner 😼