There is no best here. It is really about what works for you and what site you want to build. There are plenty of accessible WordPress Themes. The default ones usually are and accessibility is one of the sort options you can use when searching and narrowing down the themes. Short of getting someone whose opinions and taste you find to be similar to yours and that you trust with working eyes to look at some and choose one, you will probably need to load up a few and navigate through them to see how it looks/feels to use with a screen reader. If you are gonna go that route probably best to have some content on the site so you get a more realistic feel for how it will work with your content.