The "Internal" WAD. This is the base game (Doom2.wad). You must own the game to have this.
You can also find it through Bethesda Support which now offers a unified DOOM + DOOM II experience with built-in mod support. 2. Use a Modern Source Port download doom 2 wad
While you may find sites offering the original DOOM2.WAD for free, these are often legally grey or outright piracy. To play custom mods, you first need the official IWAD from a licensed copy of the game. The "Internal" WAD