I'm not going to recommend champs and might be stating the obvious.
But whatever champ you choose, don't just play 3-30 with.
Spend some time in the practice tool and speed up the muscle memory.
Some champs have situational specific mechanics that you can't exactly practice more than a handful of times in a game (lee sin "insec" style plays comes to mind)
Spending 30 min- 1 hr per week tunneling on mastering these can and will set you apart from others who play the same champ.
The practice tool allows for a greater sample size across a smaller time frame. It's the hyperbolic time chamber.
Use your downtime efficiently, find the best players of said champion and review video of matchups/itemization etc.
Gleaning through elite level footage while taking a dump is a good way to keep your brain wired to everything that is said champ without sacrificing personal life to master a champion asap.
Again this may all be obvious to most, and it boils down to the more effort you put into mastering the champ -regardless of which it is- the sooner you can master it.
Even that is only half the battle.
As you need to learn to master league as a game as well.
But the two should go hand in hand if you actually aim to get better.