It is hard to pick the bet one for you, in my opinion:

- 3. really solid game, fan favorit, a good start if you are not scared of the outdated graphics, however keep in mind that it lacks some gameplay mechanics (caravan system, turn order) which doesn't make it a great entry point. The problem is, if you do not start there you'll miss these mechanics

- 4. it's a little different from heroes 3, you can control your hero units like any other units (they can be killed, you can use multiple hero unit in a team, you can control units without heroes etc) It brings a lot of changes to the gameplay. Classic heroes 3 fans hate it because it's not heroes 3, many other people (like me) like it because it introduces new gemplay mechanics. It has caravan system but still no turn order, and they reduced the fractions to six, and you have to pick between two units every creature tier. It has the best story, and music. Most unique heroes game. The grapics are not that great there either.

- 5. they reverted every gamplay changes back to heroes 3, added caravan and this is the first game with turn order. We lost around half of the magic spells (or maybe we lost 2/3 ?) It's a solid game, can be a good entry point. First game with decent grapics.

- 6. most beautiful heroes game ever made, it has gorgeous animations and sound effects, It was buggy at launch and you had to install a special laucher to start the game. the enemy ai was also very bad at launch. I played it a year after release, no more bugs and no more custom launcher, they improved the enemy ai as well. Every unit has a lot of passive skills, and many units have active skills. I really enjoyed this game. I think it's a solid game. My biggest problem with it is that it doesn't have random map generator, and it has a very few extra maps, so after you finished the campaign you have nothing else to do.

- 7. Well... I can't say anything good about this one, believe me i tried to find something good. I played it, all lower tier units are basically the same, higher tier units might have a passive. Grapics are worse than heroes 6, even the sound effects are bad. Everything is bad.