I'm a person on the move, so I use all of those listed in your poll. Too bad I can only vote for 3, so I won't vote.
Imma list down the advantages of each one, according to my experience, just to add to the discussion.
For smartphone, the advantages are obvious: small, and easy to carry. I can always write chapters whenever I'm travelling, or in the middle of traffic while I'm stuck. Disadvantages, however, are: if you got fat fingers like mine, it's difficult to type without making spelling errors. There's also this problem where you make accidental button press if a Messenger icon pops out (hence, you'll be seen as active). But this problem can be remedied anw...I uninstalled mine.
For tablets, same advantages with smartphone but with an extra feature where you can write like in an actual notebook using a stylus, which is a plus for me because I can shoot out words faster than I can type them. Disadvantages are the same with smartphone, though a bit lesser in severity since tablets can offer bigger and spacier keyboards for fat fingers.
Whenever I do some 'on the go' writing session, I combine the use of those two. Meaning, I use my phone for references while writing on the tablet. If I can't bring the tablet, then I'll just use the phone.
For laptop, same advantages with smartphone use plus lesser consumption of electricity. Philippine electric bills can be absurdly high as we have no nuclear power plants for cheaper energy source, so we rarely use desktops. Disadvantage is, compared to phones and tablets, laptops are bulkier while moving.
For desktop, while I can't write with it on a train, desktops offer more flexibility and functions than on phone. For one, I can listen on Youtube while simultaneously typing...or listen to seminars while doing my edits. Disadvantages are electric bills, and of course, I can't bring it anywhere.
Oh, and yeah, I still consume lots of actual notebooks cause I also write on paper for my drafts.