Worst online stuff I've seen are when I'm playing games like HotS or when I used to play RTS games online.
In HotS, I always hated how players on the enemy team would always, and I mean always target the squishy player on our team every time, throughout the entire match and just taunting whenever they could. It was both annoying and very "cheese" like (as in a simple but effective strat that everyone goes for).
RTS games when I went against some randos that would ally up and just spam build a single OP unit and just steamroll, or matches like Starcraft 1-2, where you'd get bumrushed every fucking 5 minutes, because some tossers on the net love having their RTS matches last less than 5 mins (I actually consider those types to having brain damage).
These days my worst moments are in the co-op mode on HotS, because devs at Blizz decided they didn't want to make smart enemy Ai, but instead make Ai that both trolls you and always get an easy get out of fucking jail card, any time you come close to killing them (for real, close to killing tracer?, well she's gonna get a faster cooldown to zip outta dodge and everyone on our team gets magically slowed down for nearly 4 godamn seconds, no matter where we are on the map).
Step right up come on in, feel the buzz in your veins, I'm like an chemical electrical right into your brain and I'm the one who killed the Radio, soon you'll all see
So pay up motherfuckers you belong to "V"