if you want to improve fast, I'd say ADC is probably the best for you. ADC teaches you the importance of managing farming and harassing, teaching how to cs, how to push a lead, and how to position correctly. After that I'd say jungle if you're really experienced, but support / top if you want to learn either vision control or global prescence / laning 1v1. Mid I personally think has the least impact on the game rn, as much as it pains me to say it. The meta shifted away from assassins so its no longer just target the adc and hope to push a 4v5, its now mostly just a farmfest with little 1v1ing going on unless you're a lane like Azir v Ahri or a Yasuo lane. Jungle I would honestly stay away from, it teaches you macromanagement which isn't really all that important at low elos / low levels. Honestly you can carry yourself to gold / plat off of mechanics alone. Jungle is also incredibly different fromt he rest of the roles, so theres no swapping of information in between roles. you don't need to cs, you don't need to ward AS MUCH as say support, tbh all you really need to do is get lanes shoving and contest objectives.
For champs, this is my list of champs I would play starting out:
ADC: Ashe, Lucian, Jinx, Tristana
Supp: Braum, Leona, Thresh, Soraka
Mid: Ahri, Yasuo, Orianna, maybe Veigar? I don't like Viktor bc he can be p complicated for newer players
Top: Irelia, Fiora, Hecarim, Rumble, Darius
Jungle: Elise, Lee Sin, Nunu
each of those teaches you different things, while I find most of them all really fun to play too. Those are my picks of what can help carry you and teach you different things : )
