I would recommend top, because it can be at times an isolated island as long as jungler does not interfere, you then first learn how to fight 1v1 , 1v2's and then finally 2v2's...
How to cs , how to teleport and all that jazz.
Mid can quickly turn into ARAM and people will sometimes roam randomly, just throwing off your game.
Jungle is ... well jungle , it's own game.
Adc has too many things to monitor at all times, 2 enemies , his own support.
Support doesn't teach you much mechanically, but you learn map awareness , warding and fear of god... tho I guess adc has it worse these days.
Support can support anyone , if adc sucks you have 3 other champs on your team you can support.