Most people think that the northern mandarin in China is better than southern. In general, yes! I agree with that, but it still depends on person, because every province has their own local accent, so if you are looking for the best mandarin in China then you should go for person not North or south.
There is only one exception: Cantonese, their accent is really terrible because they started speaking mandarin in recently years only. Maybe in the future their mandarin will getting better.
I think that the best mandarin is no accent!