The best argument, IMO, is the beauty of nature. If it were all incidental, why would it happen to be so beautiful? Evolutionary psychologists might say we evolved to see it as beautiful because that was somehow advantageous, but I don't buy it. We'd be lucky for that to be advantageous so that we'd see all this beauty, and also it's a hard stretch of the imagination to see the sense of beauty as purely programmed; I think beauty is intrinsically linked with cosmic truth. You could also claim that animals are beautiful solely because of sexual selection, but animals aren't the only things in nature that are beautiful. And besides, with only the primary principle of the selfish gene in action, I think animals (if they would exist at all) could have turned out a lot more ugly

Another argument for God could be psychism / telepathy, etc. Telepathy and other types of psychic events are so ubiquitous that I'd imagine that most people who claim they haven't experienced any such events have dismissed them as coincidence (or chalked them up to subconscious perception or some such) because of their physicalist/scientistic worldviews and they haven't been lucid enough to be able to judge when such events are too unlikely to be coincidental, especially when they're all taken into consideration together. There have also been plenty of successful scientific experiments in parapsychology, but you don't hear of them because the scientific community ostracizes anyone who's interested in that stuff.

Anyway, the reason psychism suggests the existence of God is that, if you have experience with telepathy, you may realize that it's basically "sharing thoughts" as if your mind isn't really totally separate from another's. Other kinds of psychic and spiritual experiences reveal this oneness between all beings and events too. At the very least, you see that consciousnesses "bleed into" each other. So, my argument is that if consciousnesses aren't completely separate from each other, then chances are if you zoom out your perspective far enough you get to the unity of all beings, a single being subsuming all smaller beings as parts, and IMO that would be an adequate criterion for Godness.