Best beginner champion?

Most tanks really. Unless those that cant rely on themselves and are teammates dependent.

So no shit like braum. Rather take Garen, Malphite, Jax/Tryndamere (splitpushing mostly), Darius, Singed, Mundo.

I would suggest Garen, Mundo, Malphite mostly for tanks. There is also Sett being OP atm and relatively tanky while dealing a lot of damages.

As for AP carry, Malzahar, relatively safe due to its passive but take care, he got slow MS. He can follow up ganks easily, he can one shot with its combo if properly done, he can sustain mana on lane while dealing damages, harder to gank, he's also really great at waveclearing and maintaining a lane.

Veigar is also great, relatively safe due to its damages and his ult that can one shot in case of issues and his big cage that can stun everyone that touches the border.

Ziggs, cause he is able to securely maintain a lane without exposing himself much if at all, he's long distance and his waveclearing is quite good.

Annie and Morgana if you feel like you can touch your spells, Annie is relatiely noob friendly and can one shot as well with a full combo as well as stunning even in AoE. Morgana however relies on a skillshot, but she got a pretty good waveclearing, has a shield that makes her harder to kill/gank.

As for AD carry, easier would be Caitlyn due to her range imo. That makes her relatively safe and not so hard to play mechanically.
Otherwise, Ashe, Miss Fortune.

Ezreal is amongst the strongest and safest ADC but he's pure skillshots, if you feel like you can land your skillshots, the mechanics themselves arent that hard so go for it. He has damage, escape/engage, poke.

And jungle, Rammus, literally braindead. Zac, braindead too. Sett.

But if you wanna learn the game, dont take a "beginner" champion. Check champions, how they work, what their kit is like (their spells/mechanics) if you think they look good, even if they are hard, go for it.
You wont learn shit playing Garen all day, you'd win, but wouldnt learn much.

Faker also recommends beginners to play Twisted Fate midlane for a specific reason, learning map awareness.

I may have forgot some but you've got a clear idea of what you can play.