The stack you already know and are comfortable with. For me, that's unquestionable Groovy & Grails, but for somebody else, the answer could be completely differ

The stack you already know and are comfortable with. For me, that's unquestionable Groovy & Grails, but for somebody else, the answer could be completely different. For the auth stuff, I default to using CAS. Again, just because I know it and am familiar with it, not because it's necessarily objectively "better" than the other choices.