For Skyrim you can start with these, all found on the nexus:
- Alternate sunglare
- Ceramic/Fine Style potions
- Realistic Lightining
- The one that changes water and shores used to be called Real Water i think and the new one is just called Water, not sure but easy to find in Nexus, one of the top mods
- Skyrim Flora Overhaul
- Lush Trees/Grass
- The better sky and moons ones
- Realistic snow
Im sure im missing some but these one alone make a world of difference for almost no performance hit, if your computer can handle it get the mod that reworked the models of several things to be less sharp and better looking (also one of the "top 100 all time) and the high res texture packs.
If you are also interested on improving player/npc looks there are countless mods like better faces, high res skin, etc on the nexus.
I use Better Females by Bella and Beautiful Males along with No More Blocky Faces. You can check those three out to make faces look a little more appealing.
Also check out the ENB lighting mods on the Nexus. There a bunch of different themes you can choose from to make your game look so much more to your liking. I use Cinemascope which essentially turns my game into a grainy movie, and it's awesome.