How old? Because if it's something between 2000 - 2005 it doesn't seem that old even though it's been 20 years, or is it about titles where you look at them and say "Ah yeah... the good 1900s"? Oh is it more like those Japanese games between pixel art and 16-bits?
Think the oldest that i have actually returned to is Drop Factory from 2007, though i do a yearly run off HHS+ still sometimes and that might be older but not sure
I will put forth True Love. It was my first professionally developed hentai game, visual novel mixed with RPG stat-building. The girls were archetypes but really well realized, the game world was kinda fun and the different activities you could do each day made it feel a lot bigger than it actually was, and the sex scenes were *really* hot. They're not as explicit by today's standards, but the writing is strong and the girls were cute, and it even had some secrets. One of my favorites still.