Is the number triple-6 good, bad, or neutral in Judaism?
How about numeric variations and combinations such as 216?
I know that 18 means 'life' due to Gematria calculations.
This number has, to the best of my knowledge, no significance whatsoever in Judaism.
You can search according to gematria here but the average person shouldn't really come up with their own gematrios, generally speaking, and expect anything from them.
You will find that there are many gematrios with both good and bad implications for practically every value.
I believe that the number 666 have no significant tendency in Hebrew, and there for in Hebrew and Jewish Gematria. By checking both Hebrew genatria on http://www.c2kb.com/gematria/?word=666 and on Gematrix: http://www.gematrix.org/?word=666 shows no higher amount of results for each side.