Might and Magic I: Secret of the Inner Sanctum
Might and Magic II: Gates to Another World
Might and Magic III: Isles of Terra
Might and Magic IV-V: World of Xeen (finish Cloudside before Darkside)
Heroes Chronicles: Warlord of the Wasteland
Heroes Chronicles: Conquest of the Underworld
Heroes Chronicles: Revolt of the Beastmasters
Heroes Chronicles: Master of the Elements
Heroes Chronicles: The World Tree
Heroes Chronicles: The Fiery Moon
Heroes of Might and Magic I
Heroes of Might and Magic II: The Succession Wars
Heroes of Might and Magic III: The Shadow of Death
Might and Magic VI: The Mandate of Heaven
(Foolhardy Waywardness campaign from Armageddon's Blade)
Heroes of Might and Magic III: The Restoration of Erathia
Might and Magic VII: For Blood and Honor
Heroes of Might and Magic III: Armageddon's Blade (except last campaign, see above)
Might and Magic VIII: Day of the Destroyer
Heroes Chronicles: Clash of the Dragons
Heroes Chronicles: The Sword of Frost
Heroes of Might and Magic IV
Might and Magic IX: Writ of Fate
Notice that the HOMM2 expansion doesn't seem to fit anywhere in the timeline, but then it wasn't developed by NWC. Also, I've no idea if the HOMM4 expansions fit, but I don't think they do. The AB campaigns are largely independant but they all seem to occur after the main AB campaign (except for Foolhardy Waywardness which definitely occurs where I've placed it, per its ending). And finally, some could be easily swapped around; there's no mention IIRC of the real order in which Revolt, Master and Tree/Moon occur, but leaving Tree/Moon for last makes more sense in light of the end of Fiery Moon and how Clash of the Dragons starts. Oh and Swords of Xeen supposedly takes place right after WOX but it has some odd stuff that contradicts much of the M&M lore.