Who speaks the most standard Mandarin?

This is not a simple question because the definition of “standard” varies among different people.

I agree that the goal of “standard Mandarin” is supposed to become a general language without distinct regional characteristic and understandable by all Chinese speaking population. Therefore, personally I cannot give a specific answer to your question because in my opinion if one speaks the most “standard Mandarin”, it means you can hardly recognize his or her origin.

In reality, people generally believe people from the northern China speak better Mandarin than the southern folks. Mandarin Chinese is majorly based on the northern China dialects and thus the northern accents are natually closer to Mandarin (but not exactly same). Interestingly it is also an obstacle for northern people to speak “standard Mandarin” as per my understanding, because they often have trouble distinguishing their regional language from the standard Mandarin–they think they are speaking Mandarin but they often unconsciously blend in their dialect accent and occasionally the usage is actually not universally comprehensible.

People from the southern part of China usually face pronunciation issues in speaking Mandarin because their own accents are often quite different from the standard. But from the perspective of standard usuage, I would like to argue that sometimes southern Mandarin is even more standard in terms of universal acceptence because when they speak Mandarin, they actually switch to another language with less regional words mixed in.

In mainland China, the Mandarin in use is still heavily influenced by the north, so we often come across some northern slang that is understandable but rarely heard in the rest part of China. It is reflected in many of the posts and probably it is a hard nut to crack for Chinese language learners.