Grinding flour

That first part is a tough question.  Are you looking for more fiber?  Oats and barley are your best bet for that.  They have more fiber overall than the other grains, and a greater proportion is soluble fiber.  Rye is quite good in the fiber department also, but it is mostly insoluble fiber.  If you are trying to move along the contents of your gut, rye works quite well.  If you are pursuing the alleged health benefits of soluble fiber, oats and barley are what you want.  All three grains have distinctive flavors, and that choice comes down to what you personally prefer.  None of the three will make bread that rises high, unless you add some wheat for the gluten content.  Some people can't eat wheat gluten, and wheat is yet another flavor to consider.  Red wheat can seem too bitter to some, but others (like me) enjoy the taste.  White wheat lacks the bitterness but still contains the gluten.  The fiber content is slightly lower than that of rye, and mostly insoluble.  The only other grain I have personally tried is millet, and I cooked that whole in stews after toasting it so I don't know how millet flour would taste compared to the others.  I know it contains less fiber then any of the four above.   I haven't mentioned corn flour because that is an altogether different beast.

I don't use more than 1/6 of flours other than wheat in my bread because it doesn't rise properly if I use more.  Currently my wheat is a mixture of winter and spring.  If I were to use all spring wheat I might get away with more of the other flours.  Others here have baked loaves made completely of spelt or other ancient grains, but I have not yet had the pleasure of trying those grains.

For a bit of humor, my husband's answer was "banana flour".  It probably wouldn't make very good bread but it would surely be healthy, and my husband would like the flavor better than any of the grain flours I use.  *laugh*