Distant terrain in Skyrim is famously ugly, but we can fix that at least to some extent. Because your terrain is affected and changed by whichever mods you chose, the solution is to generate your own LOD (level-of- detail, the textures for distant terrain) based on your personal mod list.
There are two tools with which you can generate LOD. You will have to choose between them.
...is based on xEdit. What it does is create LOD files in a way similar to the Creation Kit. Distant objects that were added by mods (and are flagged with Has LOD) are now visible from a greater distance instead of suddenly popping in. In addition to that, tree LOD is also regenerated, allowing you to mix-and-match tree overhauls.
...goes a little further than SSELODGen and is thus also more performance-hungry. It does not only regenerate LOD for mod-added objects, but also adds LOD for tons of vanilla objects. That means that your distant terrain will be much more detailed and blend better with the surroundings.
A Note on Tree Billboards
The textures for distant trees are all included in one file:
Data\textures\terrain\tamriel\trees\tamrieltreelod.dds. To avoid mismatched LOD, tree overhauls always come with an edited version of that file - problem is, if you have two mods with their own tamrieltreelod.dds, they will not automatically "merge" together. The result will be mismatched LOD, affecting the mod that is loaded earlier in your mod order. To fix this, we simply re-generate LOD - including the tamrieltreelod.dds - with either SSEEdit or DynDOLOD.
For that we need billboards. What are billboards? Basically they are textures paired with INI files. I won't pretend to have any clue as to how they work or what they actually do, but it is not important in any case - you only need to know that you need billboards if you re-generate your LOD.
Fortunately, billboards were already installed in VISUALS, Module III. We are using Indistinguishable Billboards as a base (they cover vanilla trees) and then overwrite them with the billboards of whatever tree overhaul you're running. If you went through VISUALS, this should be set up correctly for you already.
SSELODGen or DynDOLOD?
DO I REALLY HAVE TO GENERATE LOD FOR MY MODLIST?
If you are using any tree overhauls, you definitely have to. If you don't it is still highly recommended to run at least SSELODGen for mod-added objects and generally improved distant terrain.
I WANT TO HAVE 3D ULTRA TREE LOD!
Only possible with DynDOLOD. You will need Enhanced Vanilla Trees as your tree overhaul and some tweaks (instructions included in the DynDOLOD section).
WHAT IF I ALREADY STRUGGLE WITH A SOLID FRAMERATE?
Go with SSELODGen, it won't be much of a hit (if at all). With lower settings (they affect the texture quality) the VRAM cost should not hurt your performance too much.
CAN I RUN THE DYNDOLOD HIGH PRESET?
This heavily depends on your graphic cards' VRAM because the High preset textures really do eat up a lot. For instance, with the full VISUALS section and DynDOLOD High installed, my VRAM usage hovers around 5-6GB.
IS DYNDOLOD WORTH THE PERFORMANCE HIT?
YES. Yes. Definitely. Really. Go try it.