Nihilus kinda makes arena about choosing 4 toons.

Lillebjørn

So Nihilus is in 70% of top 10 teams. He is a mainstay in Zaul teams, a mainstay in Rex led Zaul counter teams. You want to counter the counter? How about a Nihilus lead team? He fits in everywhere, and it's not good for variety when not including a specific character most likely is a mistake almost no matter what team you build.

Monxie

Wiggs+Chaze+R2 and "R1 teams" are good without Nihilus. But he's obviously a fantastic toon.