Mods & CC

This is very much an "It depends" situation. Mods and CC come in lots of varieties, and what you want/need won't be the same as what someone else does. Some of the varieties of mods are:

1. Bugfix mods. I post new ones when they're released in the current Broken/Updated Mods thread, which is always linked in Mod & CC Issues: START HERE. Often a bugfix mod is linked from the bug report. When I get a chance -- hopefully this summer -- I'm gong to build a comprehensive bugfix mod resource list, but when that happens depends on my work schedule, how often a huge patch gets released, etc.
2. Annoyance reduction mods. Most of these are autonomy-control mods. Others do things that, for example, make your Vampires die less easily or reduce phone calls to Sims. What's good depends on what's annoying you. 
3. Gameplay addition mods. These range from small to huge. Different players want different kinds of added gameplay. We don't know what you feel like you're missing. Hotels? More comprehensive mermaid gameplay? A full health care system? Mortgages? Sims marrying and moving on their own? (NOTE: sex and violence and other 18+ mods can't be discussed on the forums!)
4. UI mods. Some mods just change what the game looks like -- the colours of things, etc. Others rearrange the pie menu or other menus or make more things searchable. 
5. Live Mode tools. Some mods help you see and change info about Sims, or make quick outfit changes on unplayed Sims. Others even rearrange what's in the world. 
6. Build/Buy Mode tools. Some mods hide parts of Build mode or rearrange how its organized, for example.
7. CAS Mode tools. There are mods that add trait slots or add whole new traits/ambitions/preferences for your Sims. Some just change the background. You can change the number of columns in the view, or just make the CC wrenches look different.  There's even a mod that adds sliders for colours to hair.

For CC, different people find they're missing different things. Some people download different skin textures for their Sims. Others love to add hairstyles. Personally, I have alternative eyes for my Sims, plus a bunch of retro/historical clothes Iike, and some kitchen things and children's toys I felt like I was missing. You might be "missing" very different things from em.

This post has advice on how to source good mods/CC and how to avoid broken content and other problems: How to Use Mods and CC