The first would have been my first bass, a Hayman 40/40 that I put together from parts. The next was a somewhat tatty Fender Precision - the cheapest one in Musical Exchange but also the best feeling neck. It was a milestone because it was a Fender and that's what you aspired to. The next was a Warwick JD Thumb - absolutely wonderful neck, I had to have it despite the astronomical cost.
There was then a hiatus of about 20 years, and I fancied a 5 string. Tried various ones without finding one that was the equivalent of the Thumb in playability, until I happened across an eBay auction for an Antoniotsai (there were always a few back then) and thought I'd chance it, and it really was a lovely bass to play.
The one subsequent milestone was my first Sei, a headless fretless 5. Since then, other basses have matched what came before but nothing has been a real milestone, and I'm not sure that there could be another one.