Best monster girl IMO is vampire, not from a waifu standpoint but from being one. A vamp has so many ways to dominate from raw physical power and biting to literal mind control. Best waifus are the ones that still look mostly human but have enough otherness to tantalize, but that's a general thing and personality/attitude can make a huge difference. Like Cait's Caitness ruining her catgirl potential.
Eh, catgirls are pretty low tier imo.
"Humans but with an extra piece" are the most boring monster girls, might as well just be cosplay.
The best ones are ones that are significantly different enough to offer unique experiences, assuming that said unique experiences are actually positive.
Explain.
A hole is a hole, after all.
Sex with a centaur is strictly worse than banging a human.
On one side, it's literally just a horse, with a woman waving at you from like 10 feet away. Horse butt is not sexy, I will not budge on this.
On the other side, you just have the upper half of a woman, which is nice, but you could have a whole one.
You can't even do both at once, there's no traits that make sex more exciting, and you lose out on what is clearly the sexiest part of a woman (legs and thighs) for nothing gained.
Wsan does good work with futa centaurs, of course, but full female centaurs are just an awkward substitute for banging an actual horse.