The most appropriate? The one the person wants to learn. If there is no instrument they want to learn then it's not appropriate that they learn any.
In terms of technique and physically being able to play the instrument simply I'd say, in terms of difficulty, guitar, piano, violin. But, the person needs to WANT to learn to play if they are going to get anywhere.
For example, I've been trying on and off to learn recorder, tin whistle and mouth organ (all fairly “easy” instruments) for decades but I've never really got anywhere. I’d like to be able to play them but have no enthusiasm or interest really in getting down to learning how to. On the other hand, I've always wanted to play “folk style” fiddle. I got one years ago and after a few months could sort of “play” a few tunes. I am a dreadful player and wouldn't play in front of anyone unless I disliked them a lot but I really enjoy it and can play diddle a LOT better than I can play recorder, tin whistle and mouth organ.
You get nowhere learning an instrument as an adult unless you are enthusiastic and really want to.