This actually why my walkthrough focuses on slave route rather than love route, I actually prefer the love route for most characters. I used to mainly seduce Ward rather than blackmail her but ever since the Maid train that can be done on Saturday and Sunday it became impossible to justify the love route if your goal is to have as many girls in to your harem as possible. Seriously on Saturday you can have six different girls get +1 slave while the highest option for love is 2 people get +1 slut (lover) and one person get +2 slut (lover). For those that have trouble with math 6 > 4. On Sunday its even more apparent as you can have eight girls get +1 slave and one girl get +2 slave for a total of +10 slave in one event. No other event gives anywhere near that much training so its important to have all nine girls (Britney, Stacy, Ward, Nadir, Kayla, Karen, Monica, Ms. Wilson, and Patel) be on the slave/blackmail/contract route to take full advantage of the training.
That said here is what I prefer if I had unlimited time
Love route: Emma Maywood, Linda Hill, Monica Wilson, Kayla Porter, Suzie Barberry, Nikki Cross, Lydia Hart, Stacy Hudson, Rio Nakamura, Sofia Rodriguez, Rachel Wutt, Julie Chang, Aliyah Nadir, Rhea Patel, Amy Ward, Kitty, Abby Hart, Becca, Candy, Cassandra, Emily, Marsha, Hana, Katey Ward, Keera, Lexi, Mother Superior, Naomi Barberry, Raven, Val, and Zahra
Slave route: Roberta Maywood, Britney Maywood, Sara Wilson, Billie Porter, Tammy Porter, Kylie Manning, Zoe Barberry, Evelyn Hewitt, Issyr, Devin, Lydia's Mother, and Mrs. Nadir
Either route: Jane Hill, Karen Porter, Jill Sinclair, Heather West, Victoria Stein, Evelyn's assistant, Mona, Rio's mom, and Sydney
I wouldnt say underrated, I'm not a big fan of transformation either but a lot of people are fans of Jane here, I remember someone here saying they had Jane at level 9 by Saturday or Sunday I think, and that was just a normal playthrough he wasnt trying to only focus on her.