Fred for two reasons. Though I love Dobby, in the world of Harry Potter, unfortunately not many people do, like he doesn't have many friends beyond Harry, Ron, Hermione, Luna etc. Thus, though it was devastating to read/watch (and this is gonna sound super harsh) Dobby's death doesn't have much of an impact beyond those four (all of whom are also greatly impacted by Fred's death). In comparison, Fred had a huge family, and was very well liked, thus more people were negatively affected by his death and bringing him back would make life happier exponentially for many other people (further faciliated by the fact that he was a very fun, happy, funny guy!)
Furthermore, in regards to Dobby not having that many friends - there is a reason for this. He's a house elf, a group that is discriminated against within society, to the point of slavery. I may be wrong here, but there isn't much precedent in regards to free elves - does he get a job? how does he make a living? What are the workers rights for free house elves? Is he eligible for centerlink? Can he get a mortgage? Though in a perfect world, both Dobby and Fred could live long and happy lives, given their circumstances, it is far more likely that Dobby will have a pretty tough life in comparison to Fred.
So yeah, Imma pick Fred (though rip dobs)