Thinking about this game.

Any sugestions which part is best buy (easy,fun,length)?

They are all great, so I say: start from the beginning. :-)
You can choose between difficulites in all of them, they are all quite large. The last two start in rather special environments: Warriors of the North with a northern, snowy environment, centered on vikings, Dark Side features a Vampire, a Demoness, or Orc as main character - so they may be a choice of taste. (Also, Dark Side is a bit harder, because there is no tutorial at the beginning and you are thrown in more or less on your own.)
The first two, The Legend and Armored Princess (Crossworlds) start with a more standard approach.
In each of the games you can choose one of three heroes with specializations tending more to the warrior or the mage side.
I enjoy all of them and still play them over and over again.

Legend is cheap, buy it and try it, if you like it, you will get the others anyway. :D

I agree with Mondkalb that they are all great games. My favourite is KB: The Legend and it is a good starting point for the series to see if you like it. I regularly replay the games as they are such fun to play.

I like the HoMM series also.
They are different though, the HoMM series is heavier on strategy. You ususally have more heroes and you have opponents who try to conquer your cities. There are barely any quests.
In King's Bounty you have one hero with one army and no opponents. You explore the map, find treasures, fight monster troups and boss monsters, complete quests and try to get better troops. The focus is more on the role playing aspect, I think. The battles are however frequent and may require some planning (at least on higher difficulties). You can ususally choose what to fight and what not, but some areas may be blocked by monsters.

Also, besides the campaigns, in the HoMM series you normally play on a specific map. KB is more like a rpg: there is one huge world and no stand-alone maps.

I found a video on youtube on both series: https://www.youtube.com/watch?v=LJeKPQRj_Vc&ab_channel=HarvesterofEyes

King's Bounty: The Legend has by far the best quests, some are very original and downright hilarious (not gonna spoil anything). Armored Princess/Crossworlds stuck me as way more generic, and the follow-ups which I couldn't be bothered with seem to me tired rehashs of the same formula.

Compared to Heroes of Might and Magic, this game is like a much better version of the adventure campaign maps of HOMM series, i.e. the ones where you mostly explore and get troops from roadside dwellings. On top of that, since it's much longer than those scenarios (the whole game is one continuous flow), it plays like a kind of puzzle where you try to find out in which order to tackle monsters in various areas so you're strong enough to face more powerful ones, and so on. Of course your troop composition and skill in handling them in battle is very important.

HOMM series tactical scenarios have little in common with this game besides the battle system and map exploration style, honestly, they're a strategy game while this is an adventure game.

Note that the incompetent publisher has broken the Steam version with a new patch no one asked for (likely to promote their new game no one likes), and I fear they'll push the same one to GOG, so back up your offline installers while you can.

Patch for which game exactly please?

Legend

Yup. Looks like they rolled back the changes with a further update when they saw all the infuriated comments on Steam though.

In order from my favorite:
- Legend
- Darkside
- Warriors of the North
- Armored Princess / Crossworlds

As for the story, there are occasional references to the events from the previous games (except Darkside), but not really necessary to play in order.

Snap! That's exactly how I rank them too :)

Legend, armored princess (not crossworld), or darkside
Do not get crossworld unless you're expert in the series. The game disallows kiting to get the navigation map for the next island, so you're likely to fight lethal/invincible enemies all the time in order to progress.
For written quests, I like dark sides.

The stationary map guardians were honestly a great addition. They are also the same way in Darkside. Rushing all the isles, kiting everything and just looting without actually fighting always felt cheesy. This way, you at least have to fight something and not just loot the whole game and then actually start playing it.

Also, the map guardian enemies are generally the toughest fights on the island, meaning you have to fight other stuff to gain power to take them on. And fighting deadly/invincible enemies is where the fun's at. Just stomping over everything non-stop becomes boring pretty fast. It definitely helped with the flow of the game.