Don't reheat roast chicken, it's a recipe for disaster. Cook individual pieces and reheat them as necessary. Use chicken thigh. Chicken breast dries out faster than a girl's ♥♥♥♥♥ on hearing AdmiralBulldog's voice.

If you want to roast chicken, don't brine the chicken as others suggest as that simply dilutes the flavour of the chicken. The extra juices you get are just salt water, not to mention they leak out of the meat anyway on cooking. If you do want to brine, use a dry salt rub and brush off any excess salt before roasting.

If you really want chicken breasts, you can try poaching them, or doing them sous-vide then searing. It's the only way to reliably get chicken breasts that don't taste like reconstituted powdered meat, in my experience.