You can do The Doomsday Heist or The Diamond Casino Heist. Both require 2-4 people. For the older heists you need 4 people total and the Cayo heist is unplayable unless you're doing it solo.
The Diamond Casino Heist payout is by far the highest in GTA Online. The rewards for the mission add up toR$10.34 million on normal difficulty, and it can go up toR$11.37 million, if the heist is on hard difficulty.