I can think of four in particular:
1. Shell Island, which taught me that minimalism can be atmospheric;
2. Southerham, which introduced me to SR electrification;
3. Maybank to Radley Jct., which showcases the scenic strength of N scale; and
4. Charmouth, which electrified my latent interest in UK NG.
Edit - sorry, I missed the three layout limit... but I stand by my choices.