"Sufficient" is speaker-oriented word. So if you say "insufficient" you're implicitly adding "for me" unless you provide information about what is sufficient.
1. This God actually showing up right in front of me, he can take any physical form he likes. He can then show me he is God by created a small amount of matter. To prove that I'm not just seeing things he can do this in front of other people in a controlled environment. That would be sufficient evidence.
I would also accept...
2. Verified specific prophecies that couldn't be contrive. This means specific dates, unambiguous, not self fulfilling (Jewish people return to their home land because the Bible said they would for example of this), It can't be vague, and isn't having a high record of failure. I know you guys claim that you have this but looking at these claims they fall into self fulfilling, ambiguous, and/or vague as such are disqualified leaving a poor track record. Now this wouldn't get me to believe out right but it would be compelling evidence.
3. Information that the author couldn't possibly have known. For instance if we found a Biblical manuscript with E=mc^2 written in English (a language that didn't even exist yet). This would also be compelling.
2 and 3 would also require eliminating the possibility of time travel.
4. Miracles occurring especially as a direct result of prayer with a measurable consistent rate that. If we see odds showing a direct correlation that theists of a particular religion have far less negative things occur to them such as being rob being struck by lighting so forth then anyone else.
Physically impossible things happening with a direct connection to prayer such as an amputee regrowing their leg or a huge gash or sever burn disappear in moments. People claiming to be terminally ill then getting better is disqualified as we can't observe this and there are possibilities of a misdiagnosis.
A double blind study conducted by a hospital showing that prayer helped the sick and it was determined that a significant higher percent of the patients prayed for recovered. And this study could be repeated with the same results, this would be sufficient. (BTW we have conducted such a test on prayer and there was no significant difference between the test groups)
I would think this one would be easy considering your suppose to be able to work miracles through prayer.
5. If there was a religion that transcended cultural boundaries, meaning even those in isolated groups still held the same beliefs. Going further then this, finding extraterrestrial life with the same beliefs. This would give me reason to consider the possibility more seriously.
A single holy book that anyone of any language can open and read and completely understand in clear concise ways what it says. I mean I should be able to pick up that book and be able to read and understand it and the person next to me who only understands Japanese could pick up that same book and be able to read and understand it with the same clarity I could. Also if this result could be repeatably tested under controlled conditions. That might get me to believe.
Other things that while wouldn't convince me to believe but at least give me reason to think about it more would be these below, this could still be the result of human work though so it wouldn't be conclusive.
A truly noncontributory completely accurate holy book. As it stand with the Bible I find loads of contradictions and errors.
A truly unified religion, no denominations within bickering about what the holy text really says, it can only have one interpretation. I shouldn't need to have to interpret the holy text.
A religious text that consistently promoted peace and the religious followers had not committed atrocities. (This would have to be consistent) and the people of that religion truly behaved in a good manner. (In most cases my experience with Christianity has been less then stellar in this department, even when I was a theist myself.)
If you mean "undoubtable", check Fatima's miracle, as the very bright and heavily documented evidence.
Odd lights in the sky are no more evidence of God then it is of extraterrestrial visitation. This could very easily have been optical illusion resulting from atmospheric conditions.
Now I have to ask you what would convince you that you might be wrong?