What Skritter lists are good to study after completion of HSK 6? Any of the official/unofficial lists people have found particularly helpful?

I would recommend studying upper intermediate/ advanced material from textbooks such as 博雅 or成功之路. They provide useful vocab

About halfway through HSK5 I hit the point where the vocab I was picking up through daily use of Chinese was far exceeding anything from pre-made lists. (Then again, I'm not planning to take the test.)

I would also recommend textbooks, because you can buy the books and learn the words with some context. If you are proactive in your study and do your own research about words, you could also try the Chinese movie word frequency lists added by Mandarinboy. This is useful for filling any gaps, especially in oral Chinese vocabulary. The five lists contain the 5000 most common words found in movie subtitles, so there should be a decent amount of new material in there for you.

I see you're a physicist, you could even study some 物理学术语 if you fancied! The first baidu result brought me this:
http://wenku.baidu.com/view/3f40580203d8ce2f006623cc.html
This one is a bit more manageable http://zh.wikipedia.org/wiki/%E7%B6%93%E5%85%B8%E7%89%A9%E7%90%86%E8%A1%93%E8%AA%9E

roland's Chinese people you should know list is great, and still being updated - thanks roland!

http://www.skritter.com/vocab/list?list=115425147

I like jww1066's Mandarin Idioms list as well, nice and comprehensive

http://www.skritter.com/vocab/list?list=25182166

I also did metals (although stopped when it was getting a bit obscure) and place names, thanks to Bodin and clairehansell86:

http://www.skritter.com/vocab/list?list=12561164
http://www.skritter.com/vocab/list?list=58463326

Others to consider would be something like the top 3000 character list by cbjartli or Roland's traditional versus simplified characters list

http://www.skritter.com/vocab/list?list=29260793
http://www.skritter.com/vocab/list?list=302418553

My list of animals. You never know when you might get lost at the zoo.

I'm surprised you would need lists at this point. I maintain one list of words I encounter while reading/listening/watching Chinese content, during lessons, or using ChinesePod (I include the grammar and expansion sections, which the official ChinesePod lists in Skritter lacks).

The growth on that list is still much higher than the rate that I can get through them. Just about every interaction with Chinese content and/or Chinese friends lands me with words that I didn't know yet and that aren't in either the HSK or TOCFL lists.

You could always make your own list of words you don't understand from language exchanges. Might help strengthen your vocabulary (though your stats are impressive, I must say)!