I always choose to play what I currently enjoy most, thought I don't raid and do not care about rating for a very long time.
Affliction was always ok spec to play. Fairly easy to play in most content. Strong in pvp. I like the theme to slowly were down enemy by draining their power.
Demonology was often very clunky spec to play. Currently in BFA it is most interesting one. I like to see my army of demons detroying enemy. It is fun to manage demons unlike in Legion.
Destruction was originaly my go to spec since vanilla. However over time this spec became one trick pony centered around chaos bolt. So boring.