If you wanna learn to play the basics of the series, either the second game or Heroes Chronicles.
The second game's colorful nature and its heraldic, "color coded for your convenience" looks makes it easier to learn the basics of the series. It also has most of the elements that would become staples in the third game, but in smaller quantity... meaning it's less stuff you need to learn and memorize. It's also one very beautiful game in term of graphics and soundtrack, and the main campaign gives you a certain degree of freedom over what mission you want to play next, which is sadly missing from the third game, meaning not only is it a good game to learn the basics of the series, but it's also a fantastic game that typically trump the other games in the series in all aspects except for the cheer quantity of monsters, factions, spells, skills, etc.
That being said, I won't deny that when I switched from Heroes of Might and Magic II to III, I had to put the training wheels back on as while I knew the basics, I still had to learn a new set of monsters, spells, heroes, artifacts, factions, skills (and if there is one aspect where Heroes of Might and Magic III shines in it's definitely in the sheer quantity of everything it has)... plus master a couple new elements added into the game, notably the new Wait function in combat, the terrains and their effects on the battlefield, the specializations of each heroes... I suppose it can be done during Heroes Of Might and Magic III's Restoration Of Erathia campaign, but I found it easier to get used to all the new things by playing through a couple of Heroes Chronicles games, which only introduce a couple factions and elements at a time.
But yeah, Heroes of Might and Magic is a strange beast in that the third game is the meat of the series when it comes to depth of gameplay and the sheer quantity of game elements, scenarios and campaigns it has, if you had to play one game and only one game in the series, that would be it, but at the same time, since it has so many elements, it can be overwhelming for a new player who has to learn the basics in addition to everything else.
I haven't played the fifth nor the sixth games so I can't really comment on those. As for starting with the first game, the first game is simply too bare in my opinion and lacks many of the elements that were introduced in the second and third games that would become staples in the series: the spell points work differently, castles have no moat nor gates, making breaking or defending a siege more complicated for a novice, the factions are very unbalanced, the secondary skills are missing entirely, and the graphics are not as charming as in the second game. It also has a token plot meaning it is not necessary to play the first game to enjoy the second or the third games.
But my family and friends learned the ropes of the series with the second game so I might be biased.
Post edited December 23, 2013 by blueskirt42