What are some of the best digital slrs?

I have a nikon d3100

What you have is pretty top-of-the-line.

Now that you have the entry level camera, the upgrade should be a semi professional or professional camera. I would recommend d7000 with a good lens kit.

There's no 'best'. All of the camera manufacturers (Canon, Nikon, Sony, Pentax) make similar models at similar price points.

The entry level model you have is perfectly adequate - if you are not getting good results with it - it's likely down to user error. Buying a 'better' camera won't improve your images if you don't know what you're doing in the first place.

Hasselblad H-series
Pentax 645D
Canon 1D-x
Nikon D3x

That more or less covers it…