"What is the best target resolutions these days?"
Depends on what you're working with. What are you targeting, realistically? Apart from designing specifically for each interface, everything else is going to be different levels of compromise.
Starting with the lowest resolution is the best option for the highest quality, as scaling up won't reduce quality in most cases - you're displaying MORE information after all, but scaling down will.
If you don't want letterboxing, you will have to either:
1. Design your game to work with more or less information on different devices, i.e. you will see more or less of the scene.
2. Scale to fit - obviously losing quality because of non uniform scales. But you will show the same data on all devices... I'm not a huge fan of this, I think it looks horrible.
Stick to a max of 2048 atlases if you want to support the largest number of devices.