Depends on the mood you want. Some films focus on romance and intimacy more than plot.
Try these:
โข 9 Songs
โข Fifty Shades trilogy
โข Love (2015)
โข Secretary
โข Blue Valentine
โข Call Me By Your Name
โข Unfaithful
โข Basic Instinct
โข The Dreamers
โข Lust, Caution
Most of these are slow, emotional, and heavy on relationship tension. Perfect for couples who want something intimate and grown.