I think there are 2 candidates for this, and they are Walker Buehler and Gerrit Cole.

We will start with Gerrit Cole who in 84 career postseason innings he has a 2.68 ERA, 108 SO and a .881 WHIP. And since he joined Houston and New York he has 2.38 playoff ERA. And I think if the Yankees make the wildcard game he could carry them to a victory.

But the person who I believe is your best choice for a Wildcard game or a game 7 is Walker Buehler. In his young career in which every season he has pitched in the playoffs, he has pitched to the tune of a 2.35 ERA, 83 SO and a .995 WHIP in 61.1 IP. And since his blow up start in his first postseason appearance he has a 1.76 ERA and 76 SO in 56.1 IP with a K/9 of 13.5. Not to mention in a game 163 in 2018 as a rookie he went 6.2 IP and only allowed 1 hit which propelled his team to victory and a division title. Which is why I believe at the age of 27 Walker Buehler has solidified himself as the best big game pitcher in the MLB.