Battle for Wesnoth: open-source, turn-based strategy game
https://www.wesnoth.org
My only gripe with the game is that healing doesn't give XP to the healing units. This means you need to place them in combat to level up instead of placing them behind the fighters like they are intended to be, and with them initially having low health they are very squishy. I know you can kinda cheese it by reducing a monster to 1-2 HP and then getting them to attack, but it feels like going against their role.