Hard to answer that since both are consistent characters who do a decent enough job with limited panel time.
Shanks is cool, cool design, cool moments, but he's like if the OP verse had an industry plant. Oda wants me to like him very badly despite minimal panel time and when an author shamelessly pushes that agenda, it's really hard for me to genuinely think of the character as a favourite.
I prefer Mihawk, his design, attitude/demeanour is more my taste.
There are atleast 2 regulars. See some others on occasion.
I used to like Shanks more like a lot more but recently I think Mihawk is a lot more interesting
He just has so many layers like why did he decide to hunt marines, why train and tame those Baboons, he's so stoic yet has soft spot for perona and decides to cook, he's excited to be chased again yet he rejects yonko title.
I think Mihawks mannerisms, his response to different things etc have a lot of verity.
Shanks seems one dimensional now and Oda just rinse and repeated him from pre time skip
"New Era is coming" is basically Shanks entire Character summed up.
Plus Mihawk has one of the best designs in anime manga
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Like Post Time Skip made Mihawk better it made Shanks worse for me
@ZenZu