Great thread. I skipped from the first/ second page to here, so sorry if this has been covered.
Some have made the distinction between "reasonable" and "correct." I think this is the crux of the issue. There are reasonable beliefs that may turn out to be incorrect. Luminiferous aether has been mentioned: great example.
I also think that William Paley's argument from design, the "watchmaker argument" that we atheists have heard ad nauseum, was reasonable at the time Paley formulated it. There was no theory of evolution yet. It's quite reasonable then, to look at a living organism and conclude design. Even Darwin was convinced by the argument when he read Paley as a student.
You need something like a theory of evolution to refute Paley's design argument. There are other counterpoints to design (ie. how horrible, ugly, and chaotic some things are) but without evolution to put the final nail in the coffin, it's perfectly reasonable to hold the conclusion of design.
Now, those who persist with the argument today are less reasonable than Paley was in his time. There is too much support for the view that life needn't be designed. So today, holding Paley's watchmaker argument up is rather unreasonable.
Plain theistic belief is not something we can definitely disprove. Your mileage may vary from religion to religion whether it is reasonable that this or that god exists. But, speaking broadly, in principle, theism can be reasonable. Deism is more reasonable than Christian fundamentalism, for example. But is atheism more reasonable than deism? I think atheism is the stronger of the two... but I'd hardly call deism devoid of reason.