The Heroes of Might & Magic series is probably the one, apart from The Elder Scrolls, that has devoured the most of my time.
It started with HoMM3, that I was introduced to by my friends in 5th or 6th grade, and I ordered it soon after that.
After playing HoMM3 for so many hours, I thought HoMM4 would be as good, but was deeply dissapointed when almost EVERYTHING was changed, from the town-system to hero-mechanics. Therefore, my hopes for HoMM5 were somewhat lower than what should have been appropriate, because HoMM5 brought back what HoMM3 was, in 3D graphics!
I have loved almost every hour spent on this title (it far exceeds the "105 hours" listed on steam, I had the game on CD years before even joining steam), and this one is a strong "Must buy"-title!
If you like tactical turn-based strategy set in a rich fantasy world with many different factions to choose from, this is the game for you! (If you want to go a bit more retro I would also recommende HoMM3, of course ;) )