What would be your favorite 3 tracks that demonstrate how great your system sounds?

Don't have any real favorites, as most everything sounds great...at least to me...😁

Hmmm. 
1. The Riverboat Song, Ocean Colour Scene
2. Master recording of side 2, Dark Side Of The Moon, Pink Floyd.
3. Master recording of The Low Spark of High Heeled Boys, Traffic. 

Spanish Harlem-Rebeca Pidgeon - Female Vocals
El Macho-Mark Knopfler- Bass/Subwoofer Test
Come On In My Kitchen-Cassandra Wilson- Another Subwoofer Test
Seven Days-Sting- Total Dynamics/Clarity
She- James Newton Howard-Total Dynamics/Clarity
Couldn't resist 5!

That’s a tall order as we enjoy so many different types of music. Here are a few. 
Any Queen song

Ghost Rider- Make us stronger

Donna summer- I feel love 

Christopher Cross- Sailing

Warren G - Regulate 

Yes there are too many way more than practical to enumerate.  You have to listen to a wide ranging sample to fully appreciate things.