The easiest way to describe the HC series to this point is that it is one that shows growth. HC1 is slow, plodding, and does not tell a particular good story. It relies on a lot of extremely tired NTR tropes and the main character is mute to the point of not having a personality, he might as well not be there. Due to how short HC1 is it goes from 0-60 as soon as the corruption starts. It also has severe deus ex machina issues. If you like pixel animations and NTR you'll probably find some enjoyment out of it, though. It's a product with limitations, but it shows a lot of promise.
HC2's improvement over HC1 is in leaps and bounds. It looks better, sounds better, and has way more variety in content. It's got a much better setting that allows for more obvious corruption and because it is a larger and longer game it takes longer for the brakes to fully come off, though once they come off it basically just nails the corruption to the wall. HC2 has only two major flaws; which is the story not properly setting up the ending leading to a recurrence of the deus ex machina problem, and an MC that becomes incredibly difficult to root for at the end due to how unnecessarily cruel the game is to him.
HC3 is, once again, a massive improvement over its predecessor. It has a better setting, better character designs, more interesting characters, a main character who has an actual personality, and a story that does a far better job setting up its early and mid game threats with a decent amount of twists and turns, and a much wider variety of scenes. HC3 feels like it's what N_Taii was trying to do with HC1 and 2. The only problem I forsee at this point is that we might be seeing another deus ex machina ending as the story is starting to remind me of HC2's build to its ending.