Arguably, Hunter x Hunter is the closest anime to the Tower of God in terms of story. Both of these anime series focuses on test arcs. The system of Hunters in HxH is also similar to the Rankers of ToG. Many of the characters also share similar elements.

Probably the closest one in terms of the tower theme, Sword Art Online is pretty similar to the Tower of God. However, how the levels in the towers are interpreted differently. In SAO, the levels represent a video-game level while in ToG, the levels are different worlds.

The bizarre similarity between the Tower of God and Fullmetal Alchemist: Brotherhood lies in the fact that both stories need a little time to get used to. In addition, the main characters are also quite similar to each other. Lastly, the tone and pacing of both stories are very alike. However, if you’re looking for similar drawing styles or stories, you might want to check the other answers on this list.