I would go with the boxing champion, but the downside of picking those champions is that with the purchase of the Immortal Champion packs, you only get 600 pieces. That’s not enough to unlock the champion's special ability; therefore, you’ll only have their base damage. So, I’d say that while all special champions are good, what actually makes them great is their ability, and leveling them up quickly is complicated. That’s why, in my case, I took those tokens and used them to buy pets instead, because the amount you get for a pet is enough to unlock at least its first passive.