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I personally like the second game the most. The third game, due to its 3D models of Castles and characters, acquired a certain artificiality. In the first and second games, everything was made with so much love, like a painting... The hero upgrade system became a bit more complex, and a character model appeared, which is more logical but doesn't feel quite right. The Wizard Castle feels lifeless; why are there Rukh creatures with the Barbarians? The Goblins are strange, almost civilized; bone weapons suit them better. The Cyclops are unattractive (but the un-upgraded monster is a very amusing creature, it's so cute!). Whatever happened to the Wizards? It seems they were reborn into the Green Castle, merging with the Fairy Castle. There are also many types of heroes and various Capitols, which make building up the Castle take longer, but overall, the third game's heroes are good, and this is just my personal opinion. I liked the Conflux Castle and other Castles, but the Knight's Castle lost some of its charm. The Druids were replaced by Monks (the Druids were cooler). It's a pity that the Peasants became neutral units, but the situation demanded it. The Necromancer Castle is good, yes, and in principle, all the Castles in the third game are good, and almost all the units are too (except for those I criticized). I don't like the Dog units; they're kind of weird. The armored dogs are better, and their name, "Marauders," is unpleasant. And the eyes in the Dungeon are painfully evil, but the Troglodytes are fun! The Lizards are cool archers, and the Dragonflies, which live in a hollow, are like Carcassonne on a roof. By the way, it's interesting that the units can cast spells and have their own special abilities. And for the Conflux Castle, it's cool that the units that occupy the towers during a siege are different. The Elemental itself is also amusing, and the battle backgrounds are not bad either. If you think about it rationally, I think most people play the third game because it has a good balance, many Castles, territories, lots of improvements, and the graphics are decent. Also, the entire game system has stood the test of time, as well as the mods (although they don't suit me personally, although sometimes I enjoy them).
The fourth game is probably the least successful. But it has a couple of good ideas.
The fifth game seems okay too, but I haven't played it much. I didn't really like the Castles; they're beautiful, but they all feel circular. I played the alpha or beta of the fifth game, and the Castle concept itself wasn't bad. But if you don't focus on the Castles, at first, it seems that they are simpler than in the other parts, and the plot is weaker, but as a game, it's quite good.
The first game is similar to the second, except for the hero skills, spells, and the number of Castles, but it's worse (by the way, the Castles were very well drawn inside). The combat is also different (it's a funny battle). And it's good for playing quick games with friends in hot seat mode if you need to play quickly; it's perfect for that.
Well, I've written a lot, but it was fun.
So, the second game is the best. Just kidding; everyone chooses for themselves