You don't say what grade you are so its hard to recommend a piece, but at GCSE the examiners are interested in dynamics and expression as well as accuracy and interpretation. It is better to do a piece that you can perform really really well and is slightly easy, than a hard piece where you risk making errors.
Ask your music teacher what the criteria is; ie- what the examiners want and how many marks they give you for each thing. You get a ton of points just for expression and dynamics, and not that many for accuracy!! So find a piece with lots of loud and quiet bits and swoopy bits with emotion!