Performance shouldn't be too different once you are into the Sony's Super Beta HiFi machines. I believe that's any model number SL-HF400 or higher. The fancier machines tended to have more editing features and there were a couple that could record Beta 1(s) which gave higher video bandwidth and tape speed in exchange for reduced recording time. Since most people aren't making new Beta recordings, playback ability is what most are looking for, so most of the fancy features don't mean much now. That being said, a low hours SL-HF400 probably almost certainly produce a superior picture to a high hours "flagship" model.
The "Flagship" models in the USA were the SL-HF1000/2000. The 2000 has a more modern/compact design and auto tracking, but I don't think they are known to be particularly reliable compared to something like the SL-HF900/1000. Main difference between the 900 and 1000 is that the 1000 can do 1s speed recording and playback which you are very unlikely to run into out in the wild along with maybe the lack of some editing features. If I've read things correctly, the HF900 has a "detail" switch which might make the picture look better or worse that no other betamax machine had.
There were also EDV beta machines that had S-Video output which theoretically would be superior to composite output, though I haven't really seen head to head comparisons. EDV beta was bigger in Japan meaning that if you go acquire one, it's likely meant to run on 100VAC instead of 120VAC, so you'll require extra transformers or chance burning out the power supply. I'm still pretty sure any of the modern Betamax machines can be modified to have essentially S-Video output by just tapping luma and chroma before they are mixed into composite with some basic op amp circuits. I've done it with Umatic which I think does look better than composite, so I don't think it would be hard to do with beta, especially since chroma and luma are often processed on completely different boards making it likely that the tap points would be pretty obvious with an oscilloscope probing around.
You'll still need to bring your own TBC though since none of the beta machines had them. Ideally it'd be one with line and frame characteristics.
Best bang for the buck would probably be the SL-HF900, though I don't think you can say it's the "best" ever made.