It's been remarked in the past that all of League of Legend's patch history is just a long exercise in trying to keep Kassadin down. Long regarded as one of the most overpowered designs in all of League of Legends, Kassadin has long been living proof that absurd mobility is often enough to make a champion completely ridiculous, even if said champion is often dealing less damage that super minions. It's not an exaggeration to say that Kassadin is the most nerfed champion in all of League of Legends, and new players would almost certainly be amazed to learn about how insane some of Kassadin's old abilities will -- we're looking at you 2.6-second silence on a 5-second cooldown.
Well, it seems that despite every effort by Riot games to the contrary, Kassadin's time to shine has once again come. It doesn't matter that all of his abilities have been nerfed into oblivion. It doesn't matter that part of his core itemization are in the weakest state they've ever been. It doesn't matter that his ultimate is a pale shadow of what it once was. Right now, Kassadin perfectly answers a huge segment of the mid lane metagame despite all of that, and for that reason, he's a must play for any serious mid laner.
So what does Kassadin thrive against? Well, technically his best matchup is the extremely out of favor Viktor, who can't do much to stop Kassadin from running him down, but the most relevant matchup is LeBlanc. She's easily the most lane-dominant mage at the moment, but even she can't so much of anything to keep Kassadin down. His passive resistance to magic damage combined with the spammable shield on his Q makes LeBlanc's life hell in lane, as he can easily outlast her early harass and then avoid her later combos with Riftwalk. Just to make matters better, Kassadin also has the option of picking up the new Banshee's Veil, which completely shuts down LeBlanc in the side lanes where she's usually looking to roam. Combined with an excellent matchup against Katarina and very good ones against Syndra, Fizz, and Orianna, Kassadin definitely has a huge place in the metagame, one that veteran players simply can't ignore.
It's not all roses for our void-walking friend, however, as there is one champion that absolutely gives him fits at the moment, and it's not a niche one. While Kassadin's matchup against Ahri isn't his worst, it's pretty close. For comparison, Ahri has a higher win rate against Kassadin than Kassadin's traditional hard-counter, Talon. Only Pantheon is more effective at shutting down Kassadin, but that pick has its own risks. Ahri, on the other hand, is already that most popular mage in the game and is specifically picked due to how good she is in blind matchups. Thus, blind picking Kassadin is a terrible idea, and he should only be used against the champions he is strong against, i.e. every magic damage mid laner except for Ahri. Use him well, and you should have no problems taking over the Rift.