So with all the fancy kits, all of the varying ways in which you can get slaughtered by an enemy in ways that (maybe) violate the Geneva convention…
Which is the one that aggravates you the most? For me it’s McCassidy’s “stun grenade.” If you ain’t playing Genji or an area deny tank (Zarya, Sigma, D.Va, Orisa), you might as well be dead.
Ageee strongly with cass being the most annoying. You take any small amount of chip damage and he throws the nade, youre dead. Some characters like moira and kiriko can survive by burning abilities but most other squishies are dead the moment they got into grenade radius.
Reaper’s ult. It’s my personal nemesis, not helped by the fact that I often got solo-ulted with it.
Winston since as a lot of supports even if you outplay leap you’re just delaying the inevitable unless your team helps.
Being shot to death by team mates firing into Genji’s Deflect.
getting put into a game with a group when you’re entire team is random solo players - its obvious every time
For me personally, it’s ranged hitscan one-shots that aren’t restricted to ultimates. As a bonus, such massive burst damage is also the most aggravating way to see teammates die, too.
Every time I see Sojs railgun icon the killfeed. It does not even has to be me. Its always “jesus AGAIN?”.
Otherwise, I would say Hanzo, because how cheesy it feels.
Hmmm, there are a few things that come to mind.
Widow Headshot from across the map
Hanzo Headshot that wasn’t meant for you
Random Junk spam
Cassidy Sticky is kinda annoying too
Gotta be the random Hanzo headshot. I mean, I love getting them myself, but hate when it happens to me.
The worst is when you see the headshot happened from a sonic arrow, because you know that wasn’t meant for anyone in particular.
Winston because as a support all you can do is delay your death if no one is helping you.
Going from spawn to point in Illios Ruins and getting one shotted by Widow 
Sleep dart followed by a widow stepping over your body and lining up a headshot in the 5 seconds you’re literally helpless.
Nope i shoot headshots with sonic arrows to assert dominance.
Getting pushed over the cliff is the only real way to die that can make me mad.
Widowmaker by far. Everything else is not even close
Me being a support, Pharah and Mercy having non-consentual sexual relationship with me while my entire team absolutely ignores them.
Either getting picked by a sig aroung a corner, or getting outshot across the n ap by a mercy as a widow maker
It’s a tie between just stepping out the door at the beginning of a match to instantly die to a sonic arrow headshot and blindly stepping into your death from a junk trap while low health.
Turrets. Low effort annoyances that shouldn’t be in a fps