Heroes of Might and Magic III came out in 1999 and a substantial number of people have simply never stopped playing it. There are active tournaments, an enormous modding scene, and a fan-made expansion community that has outlasted several of the studios involved.
The design is unusually clean for a strategy game of its era. You explore an overworld, collect resources, build one town at a time, and fight on a hex grid where every creature stack matters. Nothing is hidden behind menus you need a manual to understand, and yet the depth is genuinely deep.
The factions carry it. Castle's knights, Necropolis's undead, Dungeon's warlocks and Tower's wizards each play completely differently, and the towns are drawn with an amount of illustrated detail that later 3D entries lost.
The music deserves a mention too - Paul Romero's town themes are still what a lot of players hear when they think of fantasy strategy.
This collection covers units, heroes and fantasy iconography. One more turn, obviously.