Rather than asking if so-and-so is the Supreme God, why not say, whoever the Supreme God is, let us call Him/Her by a name. We can use any name for that matter, but we need to agree on one such name, so it becomes easy to refer to Him/Her. The name that I associate with whatever is the Highest, the Biggest, the most Beautiful, the most Illustrious, is Krsna. This is a slightly different way of looking at things, but it may make a difference in the way you view these matters of God and Religion.
Speaking of Krsna, here is a story that is forever etched in my heart, that so beautifully illustrates the magnitude and opulence of the Creator, and shows what a small space we each occupy in this Infinite Universe.
This story is from the Chaitanya Caritmrata, written by Krishnadasa Kaviraja in 1557 AD. The full text can be accessed here.