So let’s describe God a few different ways before we try to get at an ‘ultimate’ concept of him. First a basic definition of concept as “the understanding that one has in mind.” You are a person, and you have a conception of God. Other people have their conceptions of God. This is the sense in which this answer is dealing with the question.
The first one I’ll point to is the character of God, as he’s written in the Bible. This character is all-powerful, and grants rewards or punishment according to merit. This character changes in the New Testament, and these changes mark the differences between Christianity and Judaism.
So let’s position new conceptions of God against this one. If you think about this for awhile, you realize the directions you can go in vary according to the extant religions of the world. The Christians and the Jews believe different things about God, and these differences emanate out into differences in the culture and traditions of the two peoples.
So this is a different conception of God rather than merely as a character. This I’ll simply call the belief conception. You think of God as something to believe in, rather than just as something to read and try to understand.
So to position a new conception, we need to move past belief. If we don’t, then we just end up with new beliefs, or just a new character. Note the direction we’re moving in here. It’s the metaphorical distance between you and God. First you’re reading, then you’re believing.
The new direction is knowing. This is what Christians mean by having a personal relationship with Jesus. Everyone else calls it mysticism. Rather than simply believing in God, you go meet him yourself and let him tell you all about who he is.
There is only one more place to go from knowing, and that’s to being. This is the end point of all mysticism, and takes many many forms. Western Buddhists call this satori. Once you reach the end, mysticism, the act of knowing God, stops, you step into God’s presence and share a piece of his perception for a bit, then you return back to being a person, ready to start the mystic journey anew in a different direction so you can share a different piece.
Because God will always be bigger than you are, and so no human can ever come to understand all of God. That’s the ultimate conception, God as pure mystery, the Source of All.