The dialog in Fallout 4 is by far the best Bethesda has written since perhaps Morrowind. Fallout 3 had dialog that made the game unbearable for me to play, no matter how many times I went back to it. It was like everyone was a dopey idiot, and everything was sociopathically and unintelligently polarized into extreme good vs bad, us vs them, this vs that, etc... Fallout 3 features a lot of the most incompetent writing in all of gaming history.
Skyrim had boring, non-engaging, blah dialog, that still didn't amount to anything of much note. The quests were just point a to point b, and there wasn't really any user input. Choosing between A and B is not a meaningful choice, it's a falsified and unsatisfying contrived dichotomy, which ultimately doesn't reflect the player's own disposition, and is really an annoyance to be forced into.
Fallout 4's dialog has grown beyond both those terribly-written games, and is actually bearable.
Bethesda has been putting out bad dialog for years, and gotten away with it just because of the large open-world sandbox type games they've been packaging the dialog in. This time, however, the dialog actually carries a semblance of being meaningful, by providing avenue for actual player expression - similar to the writing in Fallout 1 and 2.
Can it be improved? Sure! But this is still the best Bethesda has done in maybe 10 years or more.