Personally, I can't say who is the BEST of them all, but I can say Best of species.
For pure human, I'd say Admin Assistant. I do like office ladies for the confidence and professional angle, and she has both.
For pure furry, hands down Trash Panda. The simple fact that during the wooing she can't turn around and bash my face in, all she wants is food to fill out her fat ass, and you could classify her as a girl failure? High A tier, moving on.
For Robot, it is tougher. I like ST Patrol Droid and Catalogue. I could see myself cuddling with both of them in either a wholesome or kinky way, and I am a sucker for monitor/display heads as they can be more expressive... The only knock against Cat is the fact I had to banish goat mommy in order to get her, but it is made up for guaranteed vendor with the goods. Plus a minigame that I CAN NOT WIN (not that I want to) because I always lose either 0 or 15 seconds in because of how down bad I am, but I will always go for it when she is around.
For Demons, hands down goat mommy herself, Cult Matron. All she wants to do is a little ritual and will pamper you. I was sad when I realized that once you beat her, she gone.
Honestly despite there only being one Scaile, Sewer Frog is pretty good. Might be a little pain to see/ grind because she is a 1/3 encounter, but worth it. That tongue will work wounders on me.
For Pure Monster, it has to be the Crawler. Sure she can gut you like a fish if you attack or sprint, but she is a 50/50 encounter so you can see her often. Plus you can impregnate her, and I have a stud complex.
For Undead, I would have to say the Poltergeist. Like Crawler, she is a 50/50 encounter so you can see her often, and she is relatively easy to woo since she attacks at Far.
And finally for Monster-furry, it has to be STCP-1,471-A. Unlike Smiler, she will enter your bed without fighting and claims to be your wife. Why would I hate my wife or not include her in this list?
Edit now that v0.1.3 is out for Scaile: The Chemist. Sewer frog is still good, but I prefer Musk/Pheromones over pure Oral. If it was Oral + Titfuck, then maybe the Frog would stay.