I would rank them as follows: IV & V > III >> IX & VII > Swords > VI & VIII >> I & II
IV & V: World of Xeen are easily the series high point, pretty much perfecting the turn-based, step-based, 2D dungeon crawl.
III is similar, and almost as good, but with a nastier edge to the difficulty and no qualms about killing you off in the first town with a couple of bad rolls.
Swords of Xeen is a fan-made game in the Xeen engine, later given official sanction. It’s alright, but the level design and balancing is pretty haphazard compared to the official releases.
VI, VIII, and VIII all use the same engine, and all are pretty good, though they make some missteps in the shift to free-roaming 3D environments, and lower the party size, which has a negative effect on balance and variety.
IX completes the transition into 3D and brings the party size back up. It was rushed out the door as the company was going under, and was very buggy at release, but has mostly been rehabilitated.
I and II are of only historical interest at this point. Very primitive, and surpassed in every regard by later games.
So the 6-pack is clearly the place to start, since it includes both Xeen and VI.
Not sure if you’re looking just at the main RPG series or including spin-offs. If the latter, then Heroes of Might & Magic III is essential – one of the best strategy games of all time. As much as I love the RPGs, HoMM3 is the best thing to ever carry the Might & Magic name.
Heroes II is almost as good. A bit less refined mechanically, but superior in visuals and music. Chronicles is a bunch of extra campaigns in III’s engine. Both would be easy recommendations in a vacuum, but the content in III (GOG’s version includes two lengthy expansions) will already keep you busy for a LONG time.
I have a lot of fond memories of Heroes I, but its sequels have surpassed it in every way. Heroes IV made a nice effort to experiment with the series’ mechanics, but the results were mixed. I’d classify it as an interesting but flawed experiment, though almost worth buying just for the soundtrack. Heroes V is a mostly-successful reversion to the classic gameplay concepts, but is seriously dragged down by very constrictive map design in the campaigns (and terrible story sequences).
Crusader of Might & Magic is a completely skippable action game.
Hope that helps!