What's the best resolution for an app to support multiple devices?

I'm developing my first cross-platform game. I need a base resolution and aspect ratio in which to develop my game. I know how to scale it to work on most devices, but need a sensible base resolution.

I could do anything from the 320x480 old iPhone display to 1080x1920, but I want my first release to be on Android and I want most devices to be able to play it.

Uh, you don't want a single resolution, as I guess you probably know: don't use absolute positioning, rather use relative positioning and scale down your assets to native resolution.

If you need a minimum resolution for assets, take the largest screen size you hope to support and scale down for the rest of the devices.
Scaling down looks better than scaling up.

Generally there are several sets of assets:

One for larger screens, sized at at least the resolution of the largest target platform's screen.
Another for smaller screens sized at least the resolution of the midpoint.
Another for tiny screens sized a bit smaller.
Etc.

The goal is to never scale up, but don't down-scale too much. Too small and tearing will occur, or smoothing via the alpha channel becomes invisible and your image aliases.

I would give you numbers but I don't develop for mobile/don't know resolutions.