I don't have one best song when I'm high. I like to mix it up and different days I have different tastes.
Some Dubstep with a great bass drop is always good. So by Wiz Khalifa really zones you in. Palm Trees by Flatbush Zombies is amazing.
If you're into rock, then I highly recommend taking a listen to The Necromancer and The Fountain of Lamneth -Rush. 2112 is one of my favorites too
Here's a couple others by The Cotton Jones Basket Ride... Cool Videos. I Was Stoned By The Choir & Some Strange Rain Check'm out. Psychedelic Folk/Rock/Soul.