I know this is a nebulous question, because there are many factors that affect how speakers sound, like installation, amplification, placement, etc. Taking out all that, what is the best sounding pair of speakers you've ever heard in a vehicle?
Dynaudio Esotar. They impress me every single day. I tend to have buyers remourse often and I was really afraid I would regret these especially given their cost but I've never had a set that impresses me so much even when I'm just trying to enjoy the music and not paying attention to the speakers. They're so life-like.