In lane, one of the best counters to Tahm Kench is Vladimir.
Vladimir has constant ranged harass, and he can sustain himself as well. During an extended combat, Tahm Kench would be forced to activate his shield to survive. Tahm Kench is very good against poke due to his gray health, but he fares poorly on extended trades due to how his grey health mechanig works. Vladimir excels at both. He has a poke on a very low cooldown, allowing him to prevent Tahm Kench from regenerating a lot of health. He has sustain himself. And on an extented 1x1 combat Vladimir will win against Tahm Kench.
I believe that Lulu, Jayce, and Cho'Gath would also be able to win against Tahm Kench, but I have yet to test it to be able to know. As you can see, most of Tahm Kench's counters are ranged champions and/or have some form of health sustain.
Just don't pick anything that is melee, squishy, and has no form of health sustain... you know... Riven and Yasuo. Champions that people commonly pick for no reason.