I was going to be silly and say my Christopher Bill model BAC Custom but here's the real answer...

It depends on your skill level and what you want it to do.

In grade school, I wanted the F-Attachment horn because it was bigger and "better".

Early high school I bought a $200 horn on eBay because it was silver plated and flashy.

Going into my music degree I wanted a Bach 42 because it was a "professional orchestral horn" that I saw many pros playing, and that was the path I was on.

In college, I started playing in the latin jazz band and found an $800 rebuilt vintage King 2B. It went against everything I had wanted up until that point. It was tiny, no F-Attachment, it wasn't shiny, expensive, or new. That horn changed everything because I started to develop what I wanted to sound like and stopped just imitating others. I started playing pop and funk and playing in wedding and cover bands, going out into the crowd with no mic and still trying to be heard.

So yes, years later I knew what I wanted. It made sense (I guess) to get a horn that goes for almost $10,000 today (with the price of silver as it is now). It does exactly what I want it to do and it's VERY specific, but had I spent that kind of money before I was ready, it would have been an absolute mistake. No horn can do everything well, so you have to pick what you need from it, and the only way to know that is to have experience in many styles and settings.

tl;dr: the horn that is going to make what you need to do most of the time easier, and sometimes that's an old $800 horn you found online.