Alright, so I had myself a little taste test, lol.
Top tier:
Kerrygold - Butter flavor hits you pretty quickly and strongly. Noticeably salty. What butter should taste like.
Vermont Creamery - Rich and flavorful with a nice cultured tang, especially once it's melted.
Mid tier:
Aldi Irish butter - Solid medium flavor, faintly grassy, flavor hits more at the end. Good for the price.
Trader Joe's Cultured French butter - Sweet and creamy, a bit like Vermont Creamery (both cultured butters) but clearly not as flavorful overall.
Trader Joe's New Zealand butter - Something faintly odd in the flavor when it first hits, sweet and a bit grassy as the flavor builds, with a nice aftertaste.
Shit tier:
Minerva Dairy - Highest butterfat percentage of any I tried. Has a velvety richness, but a fairly weak 'butter' flavor. Disappointing, but might be useful if you want the texture for something.
Sam's Club Member's Mark butter - Stick of wax with a hint of butter flavor. TBH, I think that's how it is for most American butters. America got lost in the margarine era and hasn't really figured out how to make good butter.