This one is easy, yet difficult.
The basic principle is whether the bass "makes you want to play." That's going to be a combination of playability, ergonomics, and tone in two different senses - is it a tone that is pleasing to you, and is it a tone that fits with the playing that you do with others.
Now, the hard part: we and our musical contexts and interests change over time. Physical developments may alter ergonomic preferences - age, weight, weakness, or maybe getting stronger, injuries…
Musical interests may change - that P-bass with flats won't work so well in a high-energy funk-jazz thang, nor will the MTD-535 sound right covering early Neil Young. If your playing interests migrate over time, or if you are active in a lot of genres, at a high level ,then you'll probably end up with some specialist axes…or maybe not - doesn't Jimmy Johnson manage to cover everything from James Taylor to Alan Holdsworth with his Alembic 5-strings?
Then there can be value issues - an axe that you don't dig but that appreciates due to collector cachet, an axe that you got for so, so cheap that you could not possibly replace it at the same value.
Over 7 years as an active hobbyist I've probably been in and out of 35 basses, 4 vs. 5-string, fretless/fretted, different shapes, woods, price points, # of frets, string-spacing. I've learned a huge amount about what I like and don't like; certain pre-conceived notions about certain woods and electronics have been shattered, and I've learned how small things (fret wire!) can make a huge difference in sound and feel.
Also, I could simply be fussy beyond belief. I was a pro trombonist a long time ago, and talk about an instrument where you have to have a good physical connection with your axe. Plus I've always had strong interest in good sound and keep a good home sound system with lots of vinyl, so I just might be nuts.
My holy grail - one fretless and one fretted that can cover most things that I do. And I have three axes that are likely keepers:
Yamaha BB2000 fretless. Neckthrough with a lot of the same characteristics of a good Pedulla, for 1/3 the price. A little bit heavy, but with DR flats, a tremendous sound that can cover upright bass territory with some authority. The Pedulla buzz basses are phenomenal, but I can't get deal with the ergonomics of the body shape. This BB2000 isn't going anywhere else!
Lakland prototype, 34" 5-string fretless, ash/ebony, with only an MM pickup, a Nordstrand 5.4. I am way lucky to have found this axe, which is a gem. Currently using light Sadowsky flats, may go to nickel rounds at some point. A game-changer, both in terms of Lakland ergonomics (really good for me) and the fretless thing.
Sadowsky Metro 5/24 Modern, Nordstrand big singles, alder/morado. Purchased from a great upright player I used to work with as a bone player back east. After 5 years, I've yet to find anything else that seems to be so useful, and at a relatively reasonable price - I've had a few Sadowsky NYC axes in the house, and this axe has equalled or exceeded them in many ways. But will I keep it forever? I don't know - I'm starting to bump up against it's limits based on the aggressive pickup placement, the Lakland ergonomics may be better for me (body shape, compound radius, thinner neck, smaller fret wire), and am thinking about options. I have a couple of fretted axes in the house, but would like to get that down to one axe I can pretty much use for everything.
And gotta give a shout-out for three axes that could have been keepers, but for ergonomics. The Xotic J-type 5 string was not for me (big J-style body, weight), but probably the hippest electronics I've been exposed to, great workmanship, great active tone that tracks the passive tone rather than shouting "preamp!" at you. If they made the same axe with a smaller body, I'd probably be done. Next, a Music Man 5HH with ebony board - if it had 19mm string spacing, I'd have been done. Finally, the Roscoe Beck basses, 4 and 5-string, have some great design features, particularly the ability to run single/parallel/series on each pickup, independently…some serious smorgasbord of tones there..but could not get comfortable with the neck on the 5-string, along with the big J-body.
Final note - some poster above suggests that they all are fungible commodities and "the more you know, the less you care." Can't say I agree. Great players can make anything sound great, but they certainly have their preferences!
This particular sentence is to me, a strong indication of keeper status. There isn't a single aspect of this bass I that would change:
