It used to be Sera, but because of Himeji, I view her now more as an older sister. Afterall she has large tits, twintails, kneesocks, armgloves... she is butt-shit nuts, and as of S2 tortured by Ariel after being dumped by The Night King... dunno, covers most of my fetishes
Saras and Nene are also promising, but didn't have a lot of development yet. Tomonori also appearance-wise, but her character is too immature for me to find it attractive.
LOL, too many posters are butthurt these days... dunno is it because Little Busters gets animated by JCStaff or because Kizumonogatari got postponed
By the way, another show I liked but it's definitely not harem is Nazo no Kanojyo X, with a lot of revolting saliva swapping... made me wonder how could a certain group of users praise that and flame toothbrushing your sister