My 15? My current one is the one I built myself, but my previous bass with 15mm spacing was called a westwood or some such thing. When I came back from the states, my Aria 'the cat' had gone missing, I can't remember how, and I didn't have much spare money so I went down to my local guitar emporium, which was at the time PJ Walkers in portsmouth and bought this 'Westwood' bass, which I think was one of those far east branded names, that was a 5 string (my first) 15mm. Played that from 1998 to 2010, when I went mad and bought an ibanez SR1000 from Utah, and had those two basses. Then I joined basschat so obviously now have 10!
For me, 17.5 is the very limit of what I can go to, which is the narrowest I can wind the strings on the EHB to. Beyond that, it seems like a good idea but they aren't basses I pick up so they get moved on.
I have been playing a lot recently, but the last gig I picked up my old #1, the SR1605, and it was like putting old slippers on, it played and sounded so good. Made me wonder what all the other basses are for!