When annotating your own work, talk about why you used the materials you did and the strengths of your work. You should also write about areas of weakness or things you could improve. If it's a sketch remember to talk about the tonal work and the use of markings etc.
When analysing another artist's work, you should talk about what you like about it and the strengths within their work. It would also be helpful to try and link it to your own work and to talk about how you could progress in your own work with the inspiration of the artist's work.
To be honest, I don't really know if examiners really read annotations. I think the most important thing is to create as much development work as possible to show that you've really thought about the materials you're using, and doing a wide range helps a lot too, for example, pencil sketches, watercolours, collages, textiles, acrylics etc. Annotations are still really important though! Just make sure you write critically and analyse well and you should be fine.