When I got Heroes of Might and Magic 3 (from now on called HoMM3), I barely heard anything about it. When I got to play it, I was immediately hooked. I never thought that a fantasy TBS game could be this much fun - and none has taken its place in my heart.
HoMM3 is a masterpiece. It is fun (lots of it), well balanced, and there's plenty of things to do, and tactics to learn. Combat is great, exploring is always rewarding, and it's challenging enough to make you never let your guard down. It's fantastically well designed, and it brings that whole "just one more turn" vibe that is the mark of the greatest turn-based strategy games.
But what won me, above all things, is the attention to detail. In the overland map, if you pass by a fountain, you can hear the water flowing. Music changes dinamically with the terrain, creatures are well drawn and animated, and the cities make you feel like you are there. It draws you in like no other TBS game, in a way that makes everything feel more "personal". You play the game, and you keep thinking about it when you stop playing - and all the while, there's always some detail you never noticed, some new tactic to explore, one more hero to develop, one more turn to play.
The expansions add some nice features to the whole package - a new town type (bringing the city types to 9), new heroes, artifacts, creatures, a good random map generator, and enough campaigns to keep you busy for a long time.
For all that, HoMM3 is in my top list of favorite games ever - and, most likely, will be in yours too, if you're even remotely interested in a good TBS game. Make yourself a favor and get it - it is as good as it gets.