I'm assuming you are looking for something for desktop use?
If you go with the low-ish output from your mainbaord or a 9$ Apple Dongle as a DAC (expect about 1Vrms from either), the Schiit Magni Heresy is hard to beat because of its 6x high gain if you want a good amount of power for EQing or so.
If you can do with a little less, the JDS Labs Atom is of course still a great choice.
The Topping L30 perform very well too, however it could really use a proper 2Vrms input (at least).
The HD 6XX aren't terribly hard to drive, for normal listening levels all these would be fine. If you get some hard to drive headphones in the future, the higher gain would be more important. Or you couldbuy a DAC with a higher output level, but don't expect any audible improvements over something like the already transparent Apple Dongle.