Best Middle ?

play annie if youre new to the game. no need to learn hard mechanics along with macro unless u wanna be stuck in bronze

Annie very much because you can play both aggressively when safe

The best middle is mundo of course! I love mundo, hes good for all lines!

Veigar for me tends to bring me the most reliable wins. You gotta ban Zed for SURE, and you still get countered here and there, but he just snowballs SUPER hard and also can nuke people late game even if he's been very behind. So veigar or also you can't go wrong with garen TBH.

A lot of heroes are good for mid. Ashe, Annie, Veiger even Yi (I go mid with Yi a lot)
If you can target you spells easily then veiger and Annie are good. Veiger can go nuts on any enemy hero if you can continiously last hit minions with his spell (AP will go above the roof) and once built he can nuke anyone's ass.
If you can properly use Yi's alpha strike then Yi can be a brilliant selection as well.
If you got mouse controll issues and want a hero easy to control then go for Ashe

Every champion is good in it's own way.
To play and level i would say low tier midlaners like: Veigar and Annie.
To learn how to play and be good at the game at a point where u can reach platinium or diamond... I would say LeBlanc, Talon, Fizz, Ekko, Zed... What do they all have in common? Roaming potential. You roam bot, get a couple of kills and snowball and get your team ahead, and late game they can destroy the adc's in a teamfight, even if you die, you take out a major player in their team. Their constant source of damage.

For pure intent of learning how does the game work I would recommend Annie.

lux its easy to play and she does a lot of damage late game i would highly recommend

I like to play with Talon in the mid, it has a lot of burst, perfect for IK.