Final answer:
The most popular mod for Terraria is the Calamity Mod, followed by the Thorium Mod. Terraria Overhaul and Tremor Mod are also popular mods, but not as widely used.
Step-by-step explanation:
The most popular mod for Terraria is the Calamity Mod. This mod introduces a variety of new gameplay mechanics, bosses, and items to expand the game's content and challenge. The Calamity Mod has a large and active community, with regular updates and support.
Another popular mod for Terraria is the Thorium Mod, which adds new biomes, bosses, and weapons, as well as a unique progression system.
While the Terraria Overhaul mod and the Tremor Mod are also popular options, they do not have the same level of widespread popularity as the Calamity and Thorium mods.