What's your "diminishing returns" preamp choice?

It just doesn't work that way IME. The answers you get will be predictable in the sense that folks will generally say that what THEY are currently using is either 1 the best they've heard to date 2. The best for the money that they can afford or are willing to pay 3. The best in THEIR system. Really there are too many variables in listening and music preferences for the answer to ever be anything less than subjective. Recent example, listening to a vintage preamp I acquired lately I prefer to my current tube preamp in many if not all ways and a 4K preamp owned by a friend but the owner of the 4k pre prefers his overall in MY system, not his. In MY system after a while it becomes distracting and even annoying for reasons I can't put my finger on but relating to tonality. He doesn't hear this. So as Ebm points out, who can say for sure? `

get everything you need for the lowest price after proper research of your system or future system. that's head-on strategy for the largest diminishing return.
buy low sell high 'n that's all you need to do.

I've tried quite a few preamps in my time, such as Joule Electra LA 150 MK III, BAT VK 30 SE, a Sonic Euphoria transformer-based passive.

I decided I again wanted to go back to a tube pre, and I had wanted to build one for some time. So, I bought a Transcendent Sound Grounded Grid kit, using silver hook-up wire, upgraded Goldpoint stepped attenuator (with Bent Audio remote volume control, Clarity Caps output caps, and NOS Amperex 12AU7 tubes), and I enjoy this little preamp more than any of my previous ones.

It is dead quiet, with great detail and dynamics. Good performance can be had with little expenditure, and, for me, the DIY aspect adds much enjoyment to this great hobby.

Regards,
Dan

No preamp at all is my choice.

A VTL 2.5 the newer version. Good price made very good, I have had the 5.5 for. 15 years WITHOUT one problem. Lucky I guess.