I don't remember every single CS:GO match I've ever gambled on, but one that was a big upset and was a massive win for me was NiP vs Vega Squadron at the first ELEAGUE Major main qualifier in 2016. Vega Squadron ended up winning almost 16-2 again NiP, and HLTV even shows you the scale of this upset (click for match link). NiP was considered one of the top teams in the world at the time, and Vega was still on the come up at the time. I didn't bet an extremely large amount on Vega, but it was funny and surreal at the same time that NiP was choking so hard.