Whoever you want. Sure, consider folks who you think will be comfortable speaking in front of others. But it is also an act of love to invite someone to be a part of your ceremony. So let that expression of caring trump how well you think someone will read.
Consider inviting folks from each side of the family to do a reading. Or choose a longer reading that can be split in half, with two people reading. Or, for adventurous folks, consider a back-and-forth reading between two people who take turns reading stanzas of a poem, for example. Certainly makes the experience more unusual and interesting!
We create a protective bubble around ourselves. That bubble helps us survive and navigate the world. But it also becomes an inhibitor to experiencing what really is in the world, because what we end up experiencing is always through the filter of our bubble. The true experience, therefore, is influenced by our bubble.
Interestingly, the bubble keeps us from fully experiencing ourselves, as well.