easiest role to SoloQ climb with?

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.

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