As people have already said, there is no perfect resolution that will apply to all devices. That's about as concrete of an answer as you can get. The article that blackout09 linked to explains it clearly:
common misconception is that you need to target a specific resolution, like 1280x720. However, there are so many different resolutions used that it's ridiculous to pick just one. It's much better to pick an aspect ratio, then scale the game to fit that. For example, instead of 1280x720, target the 16:9 aspect ratio. (Just enter any 16:9 resolution for the window size in Project Properties.) Enable Letterbox scale in the Fullsceeen-in-browser project property, and now your game should appear correctly on any 16:9 resolution. If the display is not 16:9 then black bars appear down the sides, which helps prevent unintended display issues like accidentally seeing too much of the level or menu screen. Note if you are designing a pixellated retro-style game, you may want to use Letterbox integer scale instead.
his is the same problem faced by TV producers. There are many TVs out there using aspect ratios of 4:3, 16:9 and 16:10. If a producer films a TV show only in 4:3, on a 16:9 TV there will be gaps at the sides, or possibly even unintended off-set equipment and crew visible! Similarly, if you draw a background exactly fitting one display, then run it on another display with a slightly different aspect ratio, gaps will appear at the side or content outside the layout becomes visible. This can even happen if you simply don't take in to account the space taken up by the status bar on devices like the iPhone.
TV producers solve this problem the same way you should: everything is filmed in the widest aspect ratio 16:9. Then for other displays it's cropped down. For your games, basically you need to make sure you draw your backgrounds wider (or taller, depending on orientation) than the window size, past the normally viewable edges, to ensure no gaps ever appear regardless of the device aspect ratio.