For me that was Meridian 59. That game taught me so much about everything, network code, packets, reverse engineering, dll injection, objects and oop, cryptography, btree, winapi. I was not the first but one of the first people ever to write cheats for a, no, the first MMORPG on the planet. A friend who I've met through m59 and who is way more capable took that to the next level and wrote in game radar which would show players and monsters as dots on the map. That was years later. Another guy on the team was interested in writing a bot and is now working in robotics. So yeah those early games are great learning material.