Darksiders II has a phenomenal OST. I still listen to that regularly.
That being said the Burnout 3 music was great but obviously not an OST
Gaming
Favorite instrumental soundtrack themes are from the early Silent Hill games. The theme from SH1 and the Theme of Laura from SH2 are classics.
Favorite lyrical songs used within the game for plot: Alan Wake has characters in it who are 70s rock gods called Old Gods of Asgard, and the creators got a group named Poets of the Fall to actually write the two songs that were featured from the game and they are actually really great songs. Poet and the Muse is a plot point and Children of the Elder God is played during a really good sequence.
The Messenger has amazing music harkening back to the 8 and 16bit stuff from the MegaMan games. Underrated game in general too.
Super Mario Galaxy
I love the Final Fantasy theme, in all its incarnations. But given how much I've played X4 Foundations, the original main menu music has really taken hold in my brain. And more recently, the Kingdom End DLC main menu theme.
The Chrono Cross soundtrack is just beautiful. I particularly like the opening song "Scars left by time"