Hey there! Welcome to Complex!!
The short answer: Crops, but rotate them. Fish are great too.
Something new on Skyblock this season is the dynamic economy! If one particular item is sold drastically more, it will sell less. If it's sold for less, it will sell for more. Think supply and demand! Currently, fish are a common thing for people to sell due to their high price once sold. But because everyone is selling them, their value has gone down. Other items, such as carrots, potatoes, or netherwart, are also quite profitable but are less sold by the player base, so they're worth more in-game money.
I hope this helps answer your question!