It is not yet possible to have a Top Champion. Only a champion who has unlocked five ranks of Talents can be a Top Champion, and right now, there are only three ranks of Talents.
However, we will be adding more Talent ranks to the game in the future. Once the four- and five-star Talent ranks have been released, you'll be able to unlock the Top Champion slot for your favorite champion. Until then, you can keep collecting Talent Stones for your champions and hold onto them until the rest of the Talent ranks are unlocked.