I found the ubiquitous Google Maps to be excellent. I downloaded the area I need (the whole of England basically) for offline use, and it has very rarely let me down and I've been travelling all around England and Wales for nearly a year.
Re: OT - What is the best free road map GPS Android APP for the UK?
For getting around major cities, City Mapper app is really good. It gives options of trains buses, taxis, cycling and walking etc. and how long each will take.
It only covers London, Birmingham and Manchester in the UK.