Heart is the hardest since it's behind the rib cage, brain must be hard too since it's inside the skull. Liver and lungs would just land u in the hospital.
I never said it was easy to stab the heart (especially with the sternum partially in the way) but it is possible and that'll definitely kill you. Also I didn't say stab the brain I said your brain stem/spine which meet somewhere around the neck area - however after googling it real quick turns out you actually can stab through the skull. You would probably want to go for the pterion region which is the weakest part of the skull and only 1-4.4mm thick.
As for the liver - it stores a lot of your blood usually up to around 10% of your total blood volume. You also cannot live without a liver, though you can live with only part of one. I am not 100% sure but I think there is also an artery somewhere around there so double points.
I say lungs because... I dunno they're your fuckin' lungs lol. Don't really have a real reason for picking that one.
Also you can survive stab wounds just about anywhere I'd assume with medical intervention, but no one said anything about that? If you get stabbed in a high risk location without medical intervention you're going to die. Maybe with a bit of googling yourself you can find some more ideas of where to stab. If you're actually considering this as a suicide method I would strongly advise against this though. Very painful, difficult, and if you fuck up your life is probably going to be worse than what it was before you tried.