The only consumer lens that goes significantly beyond 1:1 without accessories is the Canon MPE-65.
The SMC Pentax 1:3.5 28mm (i.e., "K series") is particularly good reversed.
Re extension and magnification, it's just like with the lens mounted forward -- each unit of focal length you add in extension increases the magnification by a factor of 1.
With magnifications this high, at f/16 you are losing so much detail to diffraction that it isn't really worth doing. I mean you could upscale a lower magnification shot and get about the same result. At 3:1 the effective f-number is about 4 times the nominal (i.e., the setting on the aperture ring) f-number, so you want something you can use below f/8. Yes, DOF will be incredibly thin. So this is why the K28/3.5 is so good -- it's quite sharp stopped down just one click from wide open (f/4.3 or so).
You'll often read that reversing on plain extension is better than reversing onto another lens, because you aren't adding any extra glass. I believed this until I actually tested it. I've found that stacking generally gives better results, if you have appropriate lenses to use as primaries.
This is a stack shot with the K28/3.5 reversed on the #2 Pentax manual tube: