Wondering which one had it the worst off whether it be helping destroying your father/mentor, to being trapped in the realm of darkness for over 10 years, or just being a sleep for over 10 years.

They all got it pretty bad, but i would say that ven got it worst and then terra. 
Even though Aqua is trapped in the realm of darrkness, at least she is still unharmed for the most part. Terra became another person, and Ven is one step away from death.

I know this is supposed to be like the "Only pick one" questions, but I just can't pick between the three of them.

Aqua had lost both of her friends and her mentor, and she's stuck in the same place for more than 12 years (Considering that she didn't age at all, time must have move differently in the Realm of Darkness.).

Terra was consumed by the darkness, had his body possesed by Master Xehanort, He killed his mentor who also consider him as a father (Technically he didn't kill him since Master Xehanort finished Eraqus off.), His mind lingers on in his armor as his Lingering Will, He witnessed his home world being consumed by the darkness, and he has done a few evil deeds during BbS (Like stealing the heart of Aurora if I'm correct.)

Ven had been lied to about his past, had experienced the brink of death multiple times already, his darkness being forcefully removed by Master Xehanort, and he is left in Castle Oblivion as a lifeless body. (Not really, he's in a comatose state since his heart is in Sora's.)

And the few things that they have in common are their friendship was torn apart, the man whom they consider as a mentor and a father is gone, their home world has been destroyed, and they suffered a fate which they are stuck in for more than twelve years.

Terra most of things he is blamed for he didn't do it, he really tried his best to stay good, but it wasn't in the cards for him, and have to "kill" his father to protect his "brother" is pretty intense, not to mention that he have been brain raped by Xehanort over the last 10 years LOL! At least Aqua is doing "fine" in the realm of darkness knowing that the hearts of her friends are still protecting her from darkness..

Ventus screwed his heart apart, but it was his resolve since the begining to save his friends (and he thinks he did it).. also he is crashing at Sora's heart for the second time ! He probably will be fine and YOUNG!

I would say Aqua, her being trapped in the realm of darkness alone for 10 years!, not only that she watched as Terra became consumed by darkness and Ven slip into a coma, she probably feels like she is responsible considering if she didn't save Terra from falling into darkness none of this would be happening now

Considering Ven is currently safe and was shown to be content after defeating Vanitas, and a great deal of Terra's issues were self inflicted (and he seemed pretty confident he could beat Xehanort out in the BBS secret ending), I have to go with Aqua. 

Not only did she lose her home and her master, but she had to fight both of her best friends and ultimately failed to save them. Although this later did prove to be a good thing, as it kept Xehanort at bay for a little bit if time, she must have felt absolutely terrible for the 10-11 years she's been in the RoD -- all of this happening right after she was bumped up to Mastery. Worst grad present by far.