My pick would be Robin from Stardew Valley. She's just so pretty and likable. I was sooooo disappointed when I learned that you can't date her. [Stardew valley spoilers warning] So I just decided to stick with Krobus, he is my adorable roommate and I love him

God, I love his voice.. the portal series isn't exactly a dating sim or anything, but I wish I could date him.

Serena from Skyrim of course

Rouge, for me Sonic Forces was a decent game (the unlockables were shit and fuck the Classic Sonic levels through), but adding in the ability to date anyone (or almost anyone) on the cast would probably improve it.
Obviously i say Sonic Forces because of the create-a-charactor/self-insert part.