This journey is a personal one and depends on what factors you consider important. I've tried most every mouthpiece out there and the one that comes closest to the ideal Wick/singing sound without terrible endurance issues is the Mercer and Barker MB4-ET. This is the same dimensions as the GW4 but with a slightly wider rim contour.
I don't use this mouthpiece because even though I find it better than the Wick and with better endurance, it still has endurance problems compared to my pre-2018 K&G 3.5D. Even newer K&G mouthpieces do NOT work for me. This, by far, has been my "holy grail" mouthpiece even if the sound I get out of it is not 100% the sound I could get from a Wick or MB. It's not a bad sound, it just has a lot of MID whereas the Wick/MB maybe have more of the higher harmonics.
And of course, any mouthpiece you like can be made to work on the Adams if you adjust the gap properly.
Adams E3 0.6 with SS Bell
K&G 3.5D
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Founder and Solo Euphonium
San Francisco Brass Band