The best Minecraft mods?

I had been playing Minecraft for years, and recently I did a little break. Now I’ve decided to come back, but an idea to diverse my game came to my head.

I’m thinking about modding my game, but I’ve never done that and I don’t know where to start. Any tips?

That said, what are the best mods for Minecraft? I was watching some showcase videos, and some mods look terrible, they stand out from Minecraft design, etc.

I look for something that is polished, complex and works for recent versions of the game. Are there any mods that are worth attention of all time vanilla player?