Despite all the Goddesses this franchise introduced over the years i always find myself favouriting the Williams sisters, like if i could have 2 of any of the female roster it would still be them.
First off, it's well established that although they despise each other, they still can't go on without each other, so if you're in a relationship with one you're also in one with the other.
Second off, you just know that they would be competing for your attention non stop. Want to suck on Anna's big fun bags? Nina intrudes and says "Im the big sister, i get to go first". You start getting intimate with Nina and eating her out? Anna busts through the door and says in an irritated voice "Having fun without me?" then promptly kicks Nina in her face before forcing your face into her crotch.
Don't get me wrong, Lili would obviously be the best pick but the fun you would have with the Williams sisters would be priceless.