What is the most nasal accent?

Ahn, there's an element of subjectivity in the perception of a tongue as nasal-- we tend to compare the target dialect with some sort of accepted norm of that language. If raised vowels are used in a dialect of a language that wouldn't normally use raised vowels, that dialect will be perceived as "nasal" to others. (Conversely, a dialect that features vowel-lowering will be perceived as less nasal than others).

Subjectively speaking, it seems difficult for me to believe that anybody could not perceive nasality in the various Northern US English dialects; to my mind, these dialects sound aggressively nasal. Other Westerners whom I've asked about this agree.

Here's a true story:

A friend saw previews of the movie FARGO. When he watched the first scene featuring Frances McDormand, he turned to me and said,"What's wrong with her.....she talks funny. She talks through her nose." I had to explain to him that the character was from Minnesota.

My friend is a native Californian...

by both given definitions of nasel I would look to Louisiana.

For nasel like high vowels: New Orleans Y'at accent.

For main linguistic definition nasel (releasing air through the nose while speaking): the Cajun accent