I've owned my pair of MDR-V6s for 4 years now, and I absolutely adore them. They were my entry into headphone collecting and headphone repair. They sound amazing, everything is crystal clear.
I've tried a lot of headphones as well. Things like MDR-V600, V700, V900/HD, SHP9600, Superlux HD681, Grado SR60, ATH-M50X. And now I've tried HD 599s and HD 6XXs... And my V6s STILL sound better than all of them. That can't be right.
Nicer and nicer headphones close the gap, but nothing does it like my 7506s and my V6s. Some have bass that sounds as good, some have mids that sound as good, some have top end that sounds as good... nothing has the whole package.
I'm just so confused... each time I go into a pair of headphones finally expecting to have something that makes me say "wow, this is better than the MDR-V6." And then I go back to V6s, and they sound better.
To be honest unless I'm missing something (I'm less familiar with the other models in the MDR series you listed), that entire list is side-grades or down-grades*, so I'm not particularly surprised you still like them best.
(*By down-grades I don't mean objectively worse in every sense, I mean a headphone that falls into a lower performance tier than yours, and by side-grades I mean headphones that fall into a similar performance tier)
Even when you upgrade to a higher performance tier with headphones there's often trade-offs.
Since just about every other variable of headphone qualities can eventually be summed to frequency response at the ear canal, that means it doesn't matter if you're improving the bass, fixing weird treble, making a headphone sound "faster" or more detailed, you are at the end of the day ending up with anywhere from a slightly different, to very different final frequency response. That in turn reflects back onto all those other individual variables: That improved bass might mean maybe those mids don't cut through the mix quite so nicely as before. That "fast and tight" bass might mean the overall quantity of bass is a bit lower. That increased detail, might mean a brighter and more fatiguing signature that isn't as easy listening to for long hours.
So almost every switch from one headphone to another, even when jumping up price changes means "You get this, but.." and it's just a question of finding a "but" that doesn't bother you. Sometimes the "but" is a pure improvement, like the tuning being radically different but sounding better in every way, but you won't find two headphones that are identical carbon copies of each other with one simply being identical but better, there's always going to be something different about the sound and that often means some kind of trade-off.
It actually feels really special when you do find a pair of headphones that's just an overall improvement in all the areas you liked the original pair for, rather than an upgrade in 2/3 and a down-grade in the other 1/3.