Does anyone know the best way to travel from chaing mainin thailand down to southern tahiland and from phuket?

AirAsia is a good start but there are other airlines to check out.Try Thai airways,Nok Air,or One2Go(if they're still in business)Thai are more expensive,but what price convenience? The rail network is pretty good and covers all destinations you mention,although you will need to bus the last couple of hours to Phuket and Koh Samui.For all practical purposes the train terminates at Phun Phin(Surattanni) and you bus from there.Don't worry about connections tourists are well catered for and operators will ume you are going on to Pkuket/Samui and will bundle you onto a bus!.The bus system will also take you to any of the holiday resorts,is cheap and reasonably comfortable.Journeys can be a bit long winded as Thai drivers cant seem to go more than 50 miles without stopping to eat.Bus journeys can sometimes be a little bit...how can I say?.....interesting?..especially between Surat and Phuket,not much of that leg is dual carriageway so bus drivers use the invisible centre lane.I don't want to sound like an old fart but you should buckle up if you can.....To sum up.I would fly to Phuket from Chiang Mai,bus/ferry Phuket to Koh Paghan,speedboat to Koh Samui and fly up to Bangkok when you go home