Everybody suffered a little bit when there are some people who suffered the most.
Bhishma - The oath he took and got a vardaan in return was actually a curse in disguise, he led a very long life full of sorrows. Never got to start his own family.
Karna - Declined at every point insulted at almost every point because he was a SUTA though he was not and couple of curses and giving his kavach-kundala and making him emotionally weak by telling him he was the eldest pandavas when he was about to enter the war.
Gandhari - Had to bear the loss of her 100 sons.
Draupadi - Being Dis-roped in-front of hundreds of people and her husbands and pitamah standing there helplessly.
Pandavas - They never really felt peace in their life ,they had to fight for everything even with their own Grandsire and their own guru and after the war was over they got the biggest shock of their life which made them hate themselves. ( Killing of their own brother )
I think these 4 people suffered the most in my view Karna and Bhishma suffered the most but between these 2 Bhishma indeed suffer more.
Let me know if i missed anyone.