It really all depends on what you'd qualify as the best part of marriage in a game like Stardew.
Game benefits? Probably rooming with Krobus might be your best option. (Even though he's not a marriage candidate-) Why? Cause bro gives lucky lunches on occasion. A pretty good meal to stock up on, in my experience.
But... Then there is the relationship aspect. Personally, the nicest marriage candidate, I believe, is Harvey. He's very sweet and romantic, and very respectable.
But then there is a story, which marriage candidate is the best story to experience. For me, that's Shane, I always befriend him quickly in my saves cause I like him to feel better as quickly as possible. His story is very impactful to watch and is pretty iconic in Stardew.
But at the end of the day... The best marriage candidate is the one you feel the most drawn to...
...
And that's why in my current save I am married to my farm!!! Minmax to the highest degree!!!!!