I am realizing that, even at 30, I'm still younger than a lot of you guys here. For me, it was a Fender Geddy Lee Jazz Bass.
They had just come out, so this was around '97/'98. I was a huge Geddy fan and a fairly new player. Up until then, I had owned two different Ibanez basses and was using the one I had bought most recently in my first "real" band. Both of them disappeared in the mix anytime I played higher than the A string. I saw the Geddy at a music store and assumed it would be expensive, but decided to play it anyways. The high-end response was exactly what I was after. When I saw how reasonably it was priced, I nearly lost it. The blocks and binding were awesome to me. One band practice later, I went to my local store and had them order one for me. It was great. I sold it a long time ago and I wouldn't use one for my main bass today (it would be a KILLER backup to my AD Jazz though), but I still love that bass. I've pretty much been a Fender guy ever since that day. I stray every once in a while, but I always come back to them.