I disagree w/ some of the choices on offer - Bheeshma, Drona, Karna and Duryodhan. Very strongly, particularly Duryodhan. Yeah, one might complain about them being unfairly defeated/killed, but that doesn't make it sad. What was sad was warriors like Shalya who wanted to fight for the Pandavas being tricked into being dutybound to join the Kauravas instead
Anyway, in the list above, I'd have had, in addition to Abhimanyu, Ghatotkacha and Draupadi's sons, Virat's sons Uttar, Sveta and Sangha and Satyaki's 10 sons (killed by Bhurishrava). All b'cos they were so young, and in the case of Virat, there were no successors. Sangha was killed by Drona while he was carrying his father to safety - he wasn't even fighting.
If I had to include any Kauravas @ all, it would be Vikarna, Gandhari's son who had protested Draupadi's insults but didn't switch sides like Yuyutsu, and was killed by Bhima on day 14. Oh, and Karna's son Vrishasena, who was killed by Arjun in a manner similar to Kumbhakarna - first his bow was broken, then his arms severed, and finally his head.
I actually am more upset @ Ashwatthama and Kripa surviving the war, and Kripa going on to be re-instated as the Guru of Hastinapur, instead of being stripped of his ranks and exiled. That's the least he deserved, assuming that as a Brahmin, he couldn't be executed in a civilian set-up.
As for my vote, I'd vote Abhimanyu, since he left a 13 year old pregnant widow, and later a son who never knew his father.