is hunter or dk more fun for pvp? and a little bit of m+ and raids
I would pick a hunter. Hunter is tons of fun in randoms with good burst and mobility and everybody wants range for dungeons. DKs can be fun when gripping enemy healers, or spreading DoTs around, can tank, but I think hunter is pure casual fun.
DK is god-tier in epic/40 man bgs, Hunter is meh. Also, DK can tank so you should get invites to Mythic+ PUGs fairly easy… good luck getting into Mythic+ PUGs as a Hunter (Hunters are a dime a dozen).
For what you are wanting to do (casual PvP and some casual Mythic+), DK is the obvious choice. I have a geared Hunter and I still get declined by PUGs, if I were in your shoes I would pick DK just because it has that tank option.
Just my opinion and what I’ve observed while playing bgs 
that really depends on which DK you are talking about.
I dont think anyone can tell you what you will enjoy chief.
Play them both and pick what you like 
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both r great.
i would prob play DK for th necrotic strikes, as killing healers is “my thing”
but hunters have insane burst. I have a bro in my pvp community we dp BGs together, and this guy legit 2 taps people… i call him mike tyson cuz hes out there KNOCKING PEOPLE OUT COLD
hunter in leveling brackets, DK at max
bm hunter is more fun and i can tolerate bad bgs while playing mine but unholy dk is a monster dps wise.
im not a raider but isnt ranged accepted over melee for raids in bfa and both can do m+ pretty well.
it just comes down to do you want melee or ranged.