Inspired by this answer.
Is a character that is currently the host of a brain devourer, body thief style, dead? Can they be raised, resurrected?
If yes, do I have to kill the intellect devourer first?
A character who is the host of an intellect devourer is definitely not dead.
If the host body drops to 0 hit points, the intellect devourer must leave it. A protection from good and evil spell cast on the body drives the intellect devourer out. The intellect devourer is also forced out if the target regains its devoured brain by means of a wish. By spending 5 feet of its movement, the intellect devourer can voluntarily leave the body, teleporting to the nearest unoccupied space within 5 feet of it. The body then dies, unless its brain is restored wthin 1 round.
As they're not dead, they can't be resurrected. If you want to resurrect the body, you have to kill it first - of course, driving the intellect devourer out will kill it within 6 seconds anyway.
Even once you've killed them, a revifiy spell will not restore the missing brain, so, while it's not specified, the body will probably just die again. A raise dead spell won't even work - if the body is missing any important parts, raise dead just fails. On the other hand, a resurrection spell or a true resurrection spell will restore the brain, as will reincarnate.
A possible workaround not needing the Wish spell is that, since the victim is not yet dead, you may not want to kill or drive out the Intellect Devourer first. Paralyze or Petrify or otherwise render the Intellect Devourer unable to act. Then cast Regenerate on the victim to regrow his brain. Then kill or drive out the Intellect Devourer.