This is a program that is used much like GZ Doom, to modify X-COM and allow the use of various mods from simple QOL mods, ones that add new enemies and research to megamods that rewrite the entire game in a impressive way that almost makes it a whole new game like xpirates or The X-COM files.
Their forum is found here for anyone interested in checking it out: OpenXcom Forums
Even more mods organised with images for you to make your own decision and even add "sub mods" to the other megamods that are available and enhancing your experience Mod.io link
I rated this high because anything that allows you to modify games should be top tier. Especially the kind of amazing work that can be produced from modders out there. Amateur and professional alike.
Their forum is found here for anyone interested in checking it out: OpenXcom Forums
Even more mods organised with images for you to make your own decision and even add "sub mods" to the other megamods that are available and enhancing your experience Mod.io link
I rated this high because anything that allows you to modify games should be top tier. Especially the kind of amazing work that can be produced from modders out there. Amateur and professional alike.