Depends on what type of adventures you like to go on. If it were me, sliders first no matter what. For some reason I always seemed to damage the sills. Some of the things mentioned here I would classify under general maintenance that you should do anyway. So if you are talking strictly about mods: 1) sliders, 2) front & rear recovery points (whether it be full blown bumper upgrades or or just hooks mounted underneath), 3) recovery gear, 4) tires, 5) you know this list will never stop, right??
Unless you're adding a RTT or have the interior already loaded with kids & dogs, a roof rack is cool to have but I can think of 10 other things to have first. Personally, I need a roof rack with 4 kids and 4 dogs.