Well, that's up for debate, but generally anything below 500ft/min is considered good in Fly the Line. But remember that greasing it isn't always best. 

This is where touchdown zone measurement comes into play. You need to get the aircraft down in good time without slamming it into the ground.

The best landings are ones where the aircraft touches down smoothly in the middle of the touchdown zone. If you can master this, then you will gain the maximum amount of bonuses and XP for that flight.