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What are the best games to mod?
(hexbear.net)
Tabletop, DnD, board games, and minecraft. Also Animal Crossing.
Rules
The older Total War games have a ton of great mods, Medieval 2 in particular. There's a ton of amazing mods, both historical (like The Italian Wars for pike-and-shot action) and fantasy (like the LOTR mod, Third Age: Total War).
The various Paradox grand strategy games are great too.