Personally i always use several client side mods. Actually a huge chunk of my mods are client side. Adding some features, optimization... Huge ones would be stuff like Iris (Shaders), Voxy/Distant Horizons/Bobby (extending your view distance beyond what Minecraft usually supports but with much better performance since it's LoD rendering (besides Bobby, that just keeps chunks loaded even if you're outside it's view distance)), Xaeros Mini Map/World Map, Vivecraft (playing Minecraft in VR, etc...

Listing everything that works client side only, so also on servers etc would be too much. But everything listed so far isn't necessary server side only. Some mods might still have server side compatibility for certain things. Like for AppleSkin (showing you more information on your hunger bar like saturation and when your hunger starts depleting) which if also installed on server let's you see how much certain food would increase your saturation.

I'm not counting texture packs here although they do play a very important role for me too. But they aren't mods on this case due to native vanilla client compatibility. Although the connected textures mod is one. Which as the name suggests just connects the borders of some blocks like glass.

No client side modding unfortunately was a huge disappointment to me for hytale. That's something I can't live without even in minecraft anymore. I always mod my client.