Whatever stack you are already familiar with. For a more in-depth answer you might need to provide more details. For example, are the mobile clients requiring

Whatever stack you are already familiar with. For a more in-depth answer you might need to provide more details. For example, are the mobile clients requiring native application functionality, or is a mobile (or responsive) web app good enough. If they require native functionality, are they on a particular platform, or is it a mix.