There are a lot of ways to scan living tissue. What is the most detailed one to date? What kinds of resolutions can it produce? Is there a different answer for 2D vs 3D?
I assume you are asking what is the most high-resolution in vivo imaging that science has performed to date?
I think the best way to answer such questions is to explain the nature of the question.
This depends a lot on what kind of sample you are interested in for the question to address. It also depends on whether we are talking about time (temporal) or space (spatial) resolution, because there is usually a trade-off for imaging methods. Some things we can image with femtosecond resolution, and other things we can with femtometer precision.
What property do you want to look at? Is it a certain staining, such as a fluorescent marker? The topography of a molecule sitting on a membrane? The force generated by a motor protein?
Do you consider a reconstructed image (in 3D) from 2D slices to be a fair comparison with techniques that aim to directly image in 3D? Microscopy and imaging are quite advanced fields and we're only beginning to scratch the surface.
Some more considerations for the questioner: is your sample moving? Are we talking about imaging insects hovering near flowers, or anesthetized flatworms? What scale of the sample is being imaged? What are the requirements for keeping it alive during imaging? For instance, perfusing nutrients or imaging a microbe growing on a substrate normally requires the modification of your microscope in such a way that may limit your optical resolution. Imaging some things necessarily requires long working distance objectives, while other specimen may be visualized at short working distances. These have tremendous impact on resolution!
As a case study: let's say we are interested in looking at the human brain with high resolution. You can do this with high temporal resolution (using EEG) or with high spatial resolution using magnetic resonance! Or are you interested in the resolution of real time activity of the brain, as measured via oxygen delivery to regions? This is not possible with either of the above; you have to rely on functional imaging, e.g. fMRI!