Too hard to decide. My favorites often change daily, but at THIS moment .... I have a strong love for the song that Gary Chapman wrote for Amy Grant to sing (they were married then, I think) called:

"All I ever have to be".
Another favorite is:
"Love has no pride" (Bonnie Raite version)

Those are two mellow choices - for a hipper, more happening sound, I just ADORE the funk/R&B stuff that Tower of Power did.
"You're still a young man", "You ought to be havin' fun", "What is Hip"????.. I can't single just one out.

Acoustic guitar has always been a favorite sound for me and also the Fender Rhodes (suitcase) and Hammond B3 - all terrific instruments, but if I had to listen to ONE instrument all day, everyday ......

(You won't believe this)

I think it's the cello.
It has one of the largest ranges in the orchestra, and has a haunting, soothing element that pulls you INSIDE the music. When played well .....

There's ALWAYS room for cello !