So it's gotta be one of these two, and they're both more about freerolling hilarious site promos than the bets themselves:
First one is the day of the Aussie Rules Grand Final a few years back - a local betting site that doesn't exist anymore was giving $50 free bets to ANYONE on any market on the game. So got together with half a dozen friends, picked a player for Grand Final MVP each, and sure enough one of them hits paying like 15:1 or something.
Have to turn the winnings over at least once to be able to withdraw them, and these friends are degenerates, so the whole lot goes on a rugby game that night. That bet gets over the line by half a point.
Wasn't a huge win, maybe a couple grand to split between us, but for the sake of literally no investment and two hilarious sweats, it's right up there.
Second one was another local site that doesn't exist any more that did this promo on the Australian Open tennis. You back a player head to head and as long as you bet at least $25 on them, you get to spin a wheel. It lands on some amount between 50c and $10, and for EVERY ace the player you backed serves, you get that amount.
I lucked out and spun up $5 on Ivo Karlovic. In a match he ended up winning at like 26-24 in the fifth set, serving about 75 aces in the process. I swear I never wanted that match to end