For any serious development, use Detours or write your own injector using the official Windows API. For modding single-player games, use mod loaders designed for that game (e.g., SKSE for Skyrim, Script Hook V for GTA V).
Tools like DLL injectors can be used maliciously to inject malware into processes, bypassing some security software. Discussions around these tools should emphasize responsible and legal use. p100 dll injector