Recommendations by CopperSun
Which mods are the best depends on what you want to mod, of course, but these are my top recs for almost anyone. Did not list expansion mods.
Feel free to make your own list by creating a new page, calling it "Recommendations by YourName," and putting it in the Recommendations category.
Quality of Life
To-Dew - An in-game to-do list. Can set up recurring notices (like to visit the cart every Friday and Sunday).
Seasonal Digspots - Makes dig spots easy to find with fun graphics
Better Mixed Seeds - Mixed seeds can become any kind of plant. Configure for your play style. Crucial for a no-buying-from-stores run.
Generic Mod Config Menu - A graphical interface for configuring all your mods
NPC Map Locations - see where NPCs are on the map
Weekly Break Reminder - Get an in-game reminder to take a break if you've been playing for a configurable amount of time. Works better than you would think.
Simple Crop Label - Like it says, simply shows you what you're growing when you mouse over your crops.
Pretty Animals and Buildings
Any of Elle's mods, especially Elle's Town Animals.
Vibrant Pastoral Recolor - when you want to see some new colors
Stardew Foliage Redone - Pretty trees and plants
NPCs
Diverse Stardew Valley - the best portrait mod imho if you want both diversity and seasonal outfits
Mister Ginger - A fun cat NPC.
Juliet and Jessie the Joja Clerks - adds a couple interesting Joja clerks; fits in very well with Vanilla while still being fresh
Doctor Hazel - Harvey is now transwoman Hazel and infinitely more interesting
Immersion
Canon-Friendly Dialogue Expansion - Adds thousands of lines to NPCs
Joja Immersion - NPCs notice when you sell out to Joja (self-promotion)
Spouses React to 'Death' - Your partner actually cares if you land in the hospital
More Stuff
Aquatic Plants - Adds some pretty wetlands plants. Best with a river farm
Shell Collection - Collect pretty shells and turn them into pretty things.
Squirrel Collection - Squirrels!
Sailor Styles - Adds Sailor Moon themed outfits, hats, and hair
Joja Seeds - Joja sells bio-engineered crops that have various benefits and a couple that are hallmark-style gifts (self promotion)