I’d have to say objectively the best will share characteristics of extended smooth frequency response, low noise, and low distortion.
That was easy, eh? But wait, there’s more….
In addition to that features matter. Does it have the needed inputs? Outputs? Any other desirable features to help tune the sound?
Then comes aesthetics. Looks matter, as does feel. High quality preamps have controls that are a pleasure to use. Smooth, firm, functional, robust . Ahh….
I think that covers it.