Oh well, that’s a really good question. There’s a simple answer and a complicated answer…
– the cell membrane (without it, you don’t have anything to hold it all together)
– the nucleus with its genetic information, DNA (without it, the cell wouldn’t be able to make the proteins and molecules it needs to fulfill its function)
– ribosomes (this is where the proteins are made based on the code found in the DNA)
– mitochondria (this is where energy in the form of a molecule called ATP is made, without energy the cell can’t do anything)
It depends on the function of the cell! There are some cells, like red blood cells and platelets which don’t have a nucleus or any DNA at all, but they still function very well. The problem with that is that they have a very short lifespan, and can’t replicate so they just die and have to be replaced very often.