i think seasons 2 and 6 were the hardest for buffy....season 2 because she fell in love for the first time, and then lost the love of her life to a monster who looked exactly like him that she would then have to kill.
and season 6 because her resurrection in the season 6 premiere really messed her up.

ultimately i chose season 6 because i think buffy was never the same after she came back from "the dead." while she faced EXTREME difficulties in every season (ESPECIALLY season 2)- she faced the issues head on and stayed true to herself. she still seemed to be the same buffy we grew to love. but in season 6- she was so different. she was much more distant and reserved- much darker and much more detached from the rest of the world. in season 2- while the problems she faced were huge, i think buffy understood that it was normal to feel sooo hurt and to be in so much pain after everything she had been through. in season 6- buffy struggled with her feelings because she felt like she should have been happy to be alive, and she should have been grateful to be given another chance to live. in season 2 she felt like her pain and her emotional difficulties were justified, in season 6 she clearly didnt- which is why i think she struggled so much in season 6. she was not only having a hard time being in this world, but she was blaming herself for feeling so distant and so detached when in HER opinion she shouldn't have been feeling this way (but in my opinion her feelings were completely justified).

^^ You swayed my vote. I answered the question without really thinking it through first because I was distracted, and I picked season 2, for the obvious reasons. After I read your comment however, I couldn't help but change my vote. You made some excellent points there.

Love your answer jemgrl323!