Aqua, Ventus, and Terra: Who Suffered the Most? [Spoilers]

Just recapping what you said,

Terra had his body taken but his heart still remains. I'm not sure if he can get his body back from Xehanort, there are very low chances.

Aqua, to protect Terra, sent him back to the world of light with her armor and plummeted into the world of darkness, alone. She also had to give up Ventus to let him rest in peace. She's been wandering the realm of darkness for many years, and to our knowledge, has only had human interaction once (meeting Ansem the Wise.) She will most likely find herself out for KH3.

Ventus, to stop the creation of the χ-blade, destroyed Vantias and himself. The remains of his heart found home inside Sora and he will recover.

It really depends on your perspective or what you think is worse. They all gave up so much, it's hard to decide.