Access the Animus. Nine minutes of exactly why Assassin's Creed I wins so very, very hard. A steady progression from quiet and deadly atmospheric ambience to stealthy evasion to outright incredible action music.
Try playing Venice Rooftops at the same time as Ezio's Family. It works surprisingly well.
One piece of music which is truly lovely is "Peace at Forli" which first plays when Ezio and Leonardo sail from Forli for Venice and then memorably plays during Ezio's "World of Cardboard" Speech in the Bonfire of Vanities DLC and then finally makes a stirring comeback in Assassin's Creed Embers playing during Ezio's final moments and the letter he left behind. It's Tear Jerker music at its finest.
Also from Revelations comes The Forum of the Ox, which plays while you're jumping across ruined ledges to chase a boat over deadly rapids. Truly an Assassin's Creed moment.
Revelations also gives us Laboured and Lost, which plays during the game's ending, showing the end of the First Civilisation. It's truly a powerful moment and the music just heightens that. Its awesomeness must be seen to be believed.
Also the main "Welcome to Constantinople" music is incredibly melodious and light, a sense of serenity, hope and new horizons all at once, which captures Ezio's feelings in Istanbul and his late age..
III gives us Trouble in Town, the angsty, turbulent music which plays during the Boston Tea Party, and when you are chasing Charles Lee. The mission is That One Level at Full Synchronization but if you time it right, its the ultimate chase music for the ultimate chase sequence. It also plays during the credits. There's also Fight Club, catchy and fun.
The Homestead theme is also lovely in III, capturing the solitude, peace and beauty of the settlement in the forest and also sounding like a mix of Native American and European themes, without being either, symbolizing the harmony which sadly never happened.
IV gives us the delightfully Pirates of the Caribbean-esque triumphant boarding theme among many others.
Black Flag is also filled with numerous actual sea-shanties that you can collect on land, including favorites like "Drunken Sailor", "Spanish Ladies" but also obscurities like "Randy Dandy Shanty". These shanties can be played by Jack, one of your crew, like the 18th Century version of GTA Radio and when sailing in rough weather, truly makes the game a magical experience, making you bond with your crew and the surroundings and feel the excitement of being part of the Wooden Ships and Iron Men generation. Especially "Fish in the Sea" when played in a rough gale or in a storm: