I've played many erotic VNs and RPGs before, but I always seem to come back to this game, because it may just be my personal favorite and deserves a 5/5.
Before I begin; however, I have to recommend playing this game on Easy difficulty, because some parts of it will require grinding otherwise. The battle fuck RPG component of the game is fairly complex, especially for being in Ren'Py. Still, the way each individual part of the monster girl can be attacked makes for a unique RPG system that fits with the 'battle fuck' nature.
The story is pretty bare-bones, but it's coherent enough for you to create a workable plot: you're an adventurer, you travel to an island with lewd monster girls, you have to defeat the Demon Queen. This could be a problem in a game focused on a compelling story, but Monster Girl Dreams' focus is on the world-building and characters, which is good, because they're both the best parts of the game.
I really enjoy the world of Monster Girl Dreams. Because you play as an adventurer, you're the outsider, making the MC just as unfamiliar with the world as the player. A common literary trick, but when the world is interesting and is built up over time, it isn't a problem. Besides, just traversing a world filled with lewd monster girls is interesting in and of itself, and this is where we get to by far the best part of the game: the characters.
The characters are why this game gets a perfect score in my eyes. I can't find a single character I dislike: not even the un-named monster girls! They all have personality, presence, and really serve to get you invested in this new, lewd world the MC is adventuring in. In fact, the monster girls are so compelling that it makes you wonder if defeating the Demon Queen is truly the right answer. I'm definitely looking forward to see if there's a MGQ-style twist to take this game to an even more upper echelon, but in the meantime, I'll give MGD the 5/5 it deserves and move on.
To conclude; seriously, if you love monster girl games, this is an absolute must play. Just set it on Easy mode and enjoy getting attached to all the wonderful characters and fun world-building this game has to offer.