Ruff40's Ultra Pack!

This modpack is basically a good little adventure pack, with many mods introducing or improving structures to explore, and introducing several new bosses. L_Ender's Cataclysm, for example, add a ton of amazing bosses that provide a unique challenge for players.

Speaking of challenge, the Scaling Health, Improved Mobs, and Farsighted Mobs mods all add a unique level of challenge that makes the game quite fun (but maybe use keepinventory).

The highlight of this pack is probably the Essential Mod, which allows users to very easily play multiplayer with their friends.

Also the modpack has some of the best QoL and tweak mods, like Sophisticated Backpacks, Storage, and the Carry On mods for added utility in storage. There's also Nyf's Spiders, Better Combat, and Sound Physics Remastered, which really are the best. Like, trust me, they're so good. 

This is also full of rendering and optimization mods like Rubidium, Canary, Fastload, etc., in order to make running it smoother for lighter setups, and the pack should be able to run just fine with shaders with a fairly subpar setup.