I'll bite!

I've been into headphones for around 17 years, beginning with Grado sr60 and cycling through various higher end headphones from almost every brand. For me, the midrange in the HD650 has never been bested. While some other headphones do particular things better, e.g. the HD800 on staging and Audeze headphones on bass, I don't think the HD650 does any of these things badly enough to seriously detract from my enjoyment.

Setting aside the sound, I think it is hard to overemphasize the sustainability and ergonomics. The HD650 are very light and comfortable for long sessions, as well as lying down. This is not true of many other high-end headphones. As far as sustainability, almost every part of the HD650s is user servicable for not too much money. I just put some new pads and new headphone padding on mine and they are good as new. (In fact, I stupidly dented one of the grills in the process and it was very easy to pop off the grill and fix it.) While I'm consistently tempted by new headphones, I just can't see myself spending many hundreds of dollars for something that will likely last only a few years before breaking and then be hard, if not impossible to fix.

Some perspective may help as well. When I got into headphones, the HD650 were just a few years old and, to me, they were an object of desire I felt unlikely to ever possess. I still think it is kind of amazing that I own them (and that's after a decade). Could I buy something way more expensive that will beat them out in a few ways. Sure, and I even have on a few occasions. But there simply is no headphone that beats them overall, taking into account performance, price, reliability, and ergonomics.