The Might and Magic series is in a bit of disparity. Might and Magic 1 and 2 are their own storyline, 3 - 5 is yet another.
Heroes of Might and Magic establishes a new canon entirely, starting with the first, leading to the second which is then followed by Might and Magic 6, HOMM 3, M&M7 and finally M&M 8. I'd say that these, together, are the best Might and Magic games.
They are only really sequels in terms of the timeline. HOMM1 deals with Lord Ironfist's consolidation of power, HOMM2 puts Roland on the throne (or Archibald if you want to play as him, but Roland is king in M&M6 making him canon). M&M 6 deals with the Devils, who arrive during the reign of King Roland, leading to HOMM3 in which the kingdom is in tatters after dealing with the Devils, leading to M&M7 with takes place in the now restored kingdom.
M&M 8 is detached in that it doesn't really continue the Erathia saga as such, although Roland does appear. It is only the chronological sequel to M&M 7. Admittedly, M&M 7 and 8 are the only ones I've actually finished and I love both.
M&M 9, HOMMs 4, 5 and 6 are all piss. They are in an entirely different universe after Ubisoft steal the franchise from its creator. They are sacrilege.
So if you're interested in the story at all, for the complete experience you'll want:
HOMM1 -> HOMM2 (as Roland) -> M&M 6 -> HOMM3 -> M&M 7 -> M&M 8
But if you're just looking to play whatever is the best then I'd say it's too hard to pick a "best". They're all very good. HOMMs 2 and 3 are wonderful and so are M&Ms 6, 7 and 8. I haven't played much of 1-5 but they're nice and old school adventures you might want to try.