In the show? Pearl, Lapis, Jasper, Blue Diamond and Spinel in that order.
In general? Dozens. If you include anime and manga? THOUSANDS.
Steven has suffered, but not the most. Not even more than any other character including the fucking townies. He had every aspect of this project dedicated to him and he wound up being the most milqutoast piece of magic space glass installed in a mortal emotional wreck ever. Tough break little dude, at least Tumblr loves you (until they find an excuse to hate you again).
Never forget that if you think your life sucks, someone somewhere has a suckier life than you. That's why nobody can ever pass the purity test. It's better to not compare and work to make your life stop sucking if you can.