Oh boy. 1 single favorite song....I don't think I could list just one. I can probably narrow it down to a handful though. These are in no particular order...
Anathema - Untouchable (that melody and the pure raw emotion throughout, it's beautiful and never fails to make me feel something deep in my soul)
Tool - Lateralus (just 9 minutes of absolute brilliance, a masterclass in progressive music)
Type O Negative - Anesthesia (there was a time in my life where I could really relate to this song. Just the overall hopelessness and not wanting to feel anything makes for an emotional rollercoaster)
Rush - The Camera Eye (Hearing this for the first time as a 6 year old blew my little mind and made Rush my favorite band from that point on. I was the weird little kid that pretty much only listened to Rush for almost 10 years until I really started discovering new music in my teens)
Moonsorrow - Jotunheim (It doesn't get more epic than this for me. Beautiful, destructive, powerful, grandiose, emotive)
Marillion - Neverland (A powerful and emotional way to end an emotional rollercoaster of an album....The greatest album of all time....change my mind)
There are so many songs I could list here and I feel like I'm leaving out so many that are deserving of the title of favorite song... ask me again tomorrow