I think it is kinda obvious which girl I like the most.
My absolute number 1 is Red Battler. She is super confident with respect to sports, but as soon as it comes to men she is very shy and easily embarrassed. Take her second form (Val Red Battler). The very first pose (the one you get for free). She is heavily blushing, which I interpret as being a bit embarrassed to take on such a daring pose in such a daring outfit. It is my absolute favourite pose of the entire game.

My number 2 would be another Red Battler
But I took them together. So for number 2 I have Virginia. I don't have her in the game (yet) unfortunately, but I think (almost) every man has had this one female teacher to fantasise about. Now that I am a teacher myself, I still like to fantasise about certain female teachers Virginia is what I would call that kinky teacher I can fantasise about.

You guessed it, number 3 would be yet another Red Battler
But as I said before, I took them together as number 1. The third place is a bit more difficult to decide on. There are some strong contestors that just missed this spot. In (not so) random order, they are Bunny, Juliette, Arcana, Rumiko, Abraël, Leeditt and Stephanie.
In the end I decided to go for the Mermaids from world 1. Apart from the big question how someone would actually do it with them (other than oral), I have to say I have always loved mermaids. They are elegant and intelligent female creatures and absolute masters in seduction. If I had to choose for girls in the game that are unobtainable but wished were obtainable, I would say "give me some mermaids!" And preferably the one with red hair, which is a theme you can see with number 1 and 2.