The strongest I have seen is 4 blademaster/4 desert 
with Hush, Red buff, and Giant Slayer. She was doing over 10k damage per round while also applying hush and red buff burn to the entire board.
To be completely honest, Kha'zix. If you give him Seraph's or Shojin, he will be sprinting around the map at mach 5 one shotting anything he comes across unless its a warden or something, then he might have to take two hits. Speedy bug wins games my guy.
Mage Zyra
RNGesus. Bastard slaughters me every game!
Limited experience so far, but the strongest thing I've built so far was Jax with Blade master and mountain buff then Shojin and 2 AS items.
Of course, getting him to the point of near perma-dodge was tilt-inducing. Guy gets blown up by a gentle breeze... (I wanted to see if I could make it work.)
Kha'zix with 1 or 2 seraph. No contest.
Can be completely countered with PD though.
I guess there's also Master Yi who is around that level of broken and Olaf who's not quite that OP but still one of the strongest units I've seen.
It's Olaf.
Full tank zed, got warmongs, redemption and dragons claw(last oponent had a mage setup so he needed mr) He simply wouldn't die and kept makeing shadows that also made shadows When the last fight ended, i had ~10 Zeds on the field... also, he had 3electro, 3assasin and 3 summoner boost...
The most consistent build I've found regardless of board is centered around berserkers:
Renekton - just needed to fill quota on desert/berserk
Voli - same as renek but for glacial
Sion - Same as above for shadow
Mundo - Best tank/sponge, good for red buff/morello which you need to build.
Olaf - backup carry, only focus items after mundo is tanky and your true carry is built
Master yi - The centerpiece in the formation. Use a berserk glove, rageblade and X. X can be a gunblade, pd, dragons claw, QS cloak. Whatever is best against the enemy comps. Gunblade is the best overall. Only other option i've really used is dragons claw when i'm against mages/infernos.
Sivir - 2nd desert/blademaster to empower yi.
The last two spots are free tbh if you stack yi and 2 star him he'll shred pretty much anyone. I usually throw in 1 extra mystic but I wouldn't worry about starring up anyone except your berserkers. The best possible setup would include an 8th mystic and then 9th is shadow lux, just for the max shadow. So then you have a double atk (blademaster), hitting 100% harder, as a cone 100% of the time, against 50% less armor, with a solid mix of stuns, frontline and utility from your other champs. You could naturally grab 2 shadow champs as 8 and 9 if you want, but i haven't found it necessary and the same goes for 4 desert.
I've beaten every type of comp with this setup if i get to 2 star yi.
