Usually for me, the best song ever is the last one I completed ("Time Ain't Got No Grave" written Dec 3, 2021)! But if I had to choose from other earthlings musical productions, I would have to go with "Apostrophe" by Frank Zappa (it is so good, they named an album after it)!! Without any lyrics to clutter the message, it is simply an onslaught of guitar from beginning to end!!! As Ziggy Stardust would say "To be played at maximum volume!"!!!!
That is probably the most difficult question to answer that there is. It's objective, subjective, convoluted, direct, subtle, brash, weird, and wonderful all rolled into one. Which genre? Popular, rock, classical, country, big band, blues, orchestral, instrumental, vocal...? Answer is almost as varied as the diversity of life on this rock of ours.
My choice (for now): Funeral For A Friend/Love Lies Bleeding Elton John
...and that will likely change as soon as I hit "post reply"...