You can definitely start from here:
They have a complete list of a must-have for scenery and world add-ons.
My personal impression on some that I use:
- We love VFR: basically a must-have freeware if you like flying VFR in certain regions of the world where it is covered. It greatly expands the default building / objects library and localise the variety. You’ll see important landmarks such as smokestack, chimneys, industrial areas like warehouses, refinieries, cranes, antenna masts, etc
- Global AI Ship Traffic: as an airliner enthusiasts, this greatly enhance the scenery when approaching a port city like Singapore or Tokyo or Sydney. Plenty of things to look at, instead of just plain blue surface of the seas.
- Any add-ons that expand the animal flock variety - levels up the immersion, especially if you love VFR sight-seeing.
Any freeware add-ons that enhances your hometown airport & local landmarks.