I would have to say my '76 4001. For several years in the early '90s, I was trying to live the dream with music in a Massachusetts band called Jiggle the Handle. We never made any money, though, so the entire time my workhorse was a Fender Japan '62 Jazz Bass reissue.
I quit the band and pretty much quit playing music in 1995 to pursue a new dream - I moved to Berkeley, CA to go to grad school at the Institute of Buddhist Studies, and I spent the next eight years studying to become a Buddhist minister. But when both my parents passed away, I moved back to Mass. to take care of my affairs, and also hooked up with my old band-mates. At around the same time, on a routine stop to Cambridge Music, I saw this in the window:
I fell in love - I always wanted a Rickenbacker, having been hugely influenced by Roger Glover, Geddy Lee, Chris Squire, and Lemmy Kilmister, so I snagged it for around $800 (unthinkable now?!!) I used it a lot in the studio and onstage a lot that summer:
It's actually not the first instrument I bought during my musical resurgence - that would be the Alembic Exploiter:
But I just sold the Exploiter this past week - I've done some recordings with it that I love, but I don't have the emotional attachment with the Exploiter that I do with the 4001. Also, my future wife was with me the day I found the Ric, maybe that has something to do with it. Even though it has its problems, I don't see myself parting with it. I don't play it all the time either, but between the appearance, sound, playability, and the fact that it is an instrument that I aspired to my whole musical life but never though I would ever actually hold in my hands, I periodically go back to it for musical inspiration, so this is the one instrument I have that could qualify as the "love of my life" instrument.