Galiois pretty effective as an assassin counter... I have heard stories of people using him as their Yasuo counter. His natural tankiness means he can shrug off the attacks of most assassins... Winds of War (his Q) is an effective ranged small radius AoE attack... Perfect for hitting Akali in her shroud for example. His W (Shield of Durand) not only provides extra tankiness from the shield, but the second cast is a taunt, which effectively stops most melee assassins (and interrupts channeled abilities like Katarina's Death Lotus). His E (Justice Punch) provides some additional Crowd Control and works as a gap closer. Finally, his R (Hero's Entrance) allows him to partially keep up with a roaming enemy assassin and provide chances to save an ally who gets ganked.
Kassadin - This champion has a lot of good counters hopefully. He has only 41.8% winrate against Wukong and then he is quite bad into champions like Talon, Nocturne, Qiyana, Renekton and Galio. Against those 5 champions, Kassadins winrate is not higher then 47%.
Talon - Pretty bad into Pyke - only 45.9% winrate... other champions like Wukong, Neeko, Katarina are also solid counterpicks but it makes Talon's winrate only slightly below 50%
Katarina - Pretty hard to counter. Against Fizz she has 48.7% winrate, everything else is just irelevant.
Alternatively, Pantheon works fairly well against assassins as well. He has point-and-click CC with Shield Vault, the ability to negate the biggest part of all-ins with Aegis Assault and he can follow up on roams with Grand Starfall.
Zed - Currently Zed has only 40.8% winrate against Wukong, 42.6% against Zilean and 42.9% against Malphite
Fizz - A bit trickier one - Currently has only 47.2% winrate against Neeko and 47.3% against Galio
Diana - Again a bit trickier one - Against Renekton, Ekko, Cassiopeia and Galio, Diana has ~49% winrate
Ekko - Pyke, Kassadin, Talon and Neeko are all OK against him. Ekko has ~48% winrate against them
Kassadin - This champion has a lot of good counters hopefully. He has only 41.8% winrate against Wukong and then he is quite bad into champions like Talon, Nocturne, Qiyana, Renekton and Galio. Against those 5 champions, Kassadins winrate is not higher then 47%.
Talon - Pretty bad into Pyke - only 45.9% winrate... other champions like Wukong, Neeko, Katarina are also solid counterpicks but it makes Talon's winrate only slightly below 50%
Katarina - Pretty hard to counter. Against Fizz she has 48.7% winrate, everything else is just irelevant.
Pyke - LeBlanc is reaaally good into him. Against LeBlanc he has only 44.6% winrate - other champions like Fizz, Vladimir or Lissandra are also very good.
LeBlanc - Pretty bad at the moment. Her giggest counters are Annie and Malzahar, however there are many champions that she is bad against, like Galio, Diana, Ahri, Talon, Katarina or Twisted Fate.
Rengar - Rammus, Master Yi, Wukong or Xin Zhao are all solid against him.
Master Yi - sux very bad against Wukong - only 43.9% winrate. Udyr, Kha'Zix or Elise are all solid too!
Kha'Zix - Ivern all da wae!
Shaco - There is no champion Shaco would be very bad against. Play whatever you can play the best.
Wukong - Karthus and Evelynn are both good champions against him - he only has 43.7% winrate against them.
Evelynn - Rek'Sai seems to be her biggest counter (43.7% winrate) but other champions like Rengar, Rammus or Warwick are good too.
Well I hope I clarified most of the assassin matchups feel free to ask If I missed some. Have a good day <3
Alternatively, Pantheon works fairly well against assassins as well. He has point-and-click CC with Shield Vault, the ability to negate the biggest part of all-ins with Aegis Assault and he can follow up on roams with Grand Starfall.