Here's my list, after giving it some major thought....(i struggled with some of these, switched some around numerous times...debating with myself...lol....)
1. Death of Uncle Ben
2. Seeing the 911 tragedy (this was a life changing sight!!)
3. Death of Gwen Stacey
4. Death of Captain Stacey
5. Being buried alive for 2 weeks (Kraven)
6. The unmasking (still can't feasibly see why Pete would do this if written properly...)
7. Sins Past, finding out Gwen slept with Osborn...(bleh!)
8. Harry's Drug Use
9. Kept in a Psych Ward (mad dog ward) and restrained, drugged, etc...
10. Aunt May's Death(s) and many sickness's and/or coma's...(lol..)
(Aunt May's death, could be higher on list but with first one retconned, and currently unknown were the present coma is going, i am leaving it at #10 for now, but would be much higher depending on ramifications if were to REALLY happen. Regardless, aunt may being ill/hurt, etc has ALWAYS been a major concern in Pete's life.)
Wow....no "Night of the Goblin"....
Here's mine:
1.
Uncle Ben's death (duh)
2.
Finding out he's a clone - Finding out your entire existence is a lie SUCKS. Especially when you've established a good life and stolen that very life from essentially, yourself. This may lead one to do crazy things like accidentally lash out at his wife, leave his life, and join an insane villian in his quest to create a world full of clones. Worst of all, NO BRAINWASHING. He was a wreck....
3.
Gwen's death (duh)
4.
Night of the Goblin - imo, this should be a no-brainer (again, imo). If MY baby died in a stillbirth the same day my brother gets killed trying to save my life from my long-thought dead worst enemy (who killed my girlfriend and indirectly killed my best friend), then I'd feel pretty ****ty. In fact I'd be SO mad, that I'd throw a bag of pumpkin bombs with enough firepower to take out a skyscraper at my worst enemies chest, surely killing him. One could make the argument that this is worse than Gwen's death but it will never be because most fans hate the clone saga while Gwen's death ushered in the Silver Age of Comics. One could also argue that this is worse than finding out he was a clone because (as established in Revelation #1) he's truly happy now before the Goblin comes and takes it all away. In a matter of seconds, he makes a decision to kill a man (osborn) and just because Osborn's rediculous healing factor saves him doesn't absolve Peter from the decision he made. Peter didn't know he was gonna live...
5.
"Death" of MJ - Living with Robbie's son (not so bad, but he's no MJ

t

, living in denial for months, being homeless and too proud to go home to Aunt May. Pretty crappy if i say so myself.
6.
Death of Harry - death of his best friend
7.
Fake Parents - You know, the whole "I am the Spider" crappened here. He went nuts.
8.
Back in Black - Outs himself, putting his family in constant, permanent danger which results in his Aunt getting shot by a sniper.
9.
Jean De Wolffe - Friend is killed by the guy you've been working with to find her killer. Sucks.
10.
Death of Captain Stacy - Your only father figure is killed and your girlfriend officially hates Spider-Man, your alter-ego, forever.
Phew...that took awhile. What a crummy life.