Any anime that try to deal normally more adult-wise with relationships. Where the characters start dating from practically the beginning of the show, there's kissing and sex. No moe or too childish situations.
I always say the best two romances, in any medium, are Kemonozume and Take on Me (porn mango).
Once you watch Kemonozume and read and fap to Take on Me, no romance will be good enough. In any medium.