Dota 2 terrains
A terrain replaces the map itself. It is the biggest visual change you can make and the usual reason somebody starts modding at all.
The D2PFX catalog has 21 mods in this category. Each changes what you see on your own screen; everyone else sees their own game.

Crownfall

Dota+ Autumn

Dota+ Spring

Dota+ Summer

Dota+ Winter

The Kings New Journey

TI10 Sanctum of the Divine

TI5 Desert Terrain

TI6 Immortal Gardens

TI7 Reefs Edge

TI8 Emerald Abyss

TI9 Overgrown Empire
Simple Dark Terrain Minify
Simple Dark Terrain MinifyGreen

Bedrock

Dark Terrain Minify

DarkPoly

DarkPoly & Trees

LowPoly

LowPoly & Trees

Mossy Cobblestone
Installing any of these
Open Dota 2 Mod Manager, find the hero, press Install. The app puts the file in a free pak slot in the folder your game mounts, tells you when two mods carry the same file, and puts everything back after a Dota patch. The full guide, or the manual route if you would rather do it yourself.