Grains in general are tough to digest. Vegetables are tough too. I found that the easiest things to digest were animal products such as fish, liver, eggs, cheese, et cetera. Violet's first food was salmon. We did a couple veggies, but they were a bit rough on the digestive end. She did a lot of eggs early on and had some fruit as well. Soft fruits are good. You can certainly start with whatever you like as long as it's soft enough or blended well enough, but if you want to go with what will nourish their bodies the most, I'd go with meat products.
Again, you can really start anywhere, so choose whatever sounds right to you. Just because meat products and other animal products feel right to me for nutrition reasons, doesn't mean it feels right to you for other reasons.
Edit: oh, and avocados and bananas were also great.