I was just researching this because a friend asked, and honestly, the ones with the biggest numbers aren't always the most visible. The Reedsy platform runs a weekly contest with a $500 prize, which adds up if you're consistent, but the submission window is tight. For sheer scale, the Self-Published Science Fiction Competition used to have a $10,000 grand prize funded by sponsors, though I think the structure changed recently. A lot of the 'big money' contests seem to be tied to specific genres or very niche themes from private foundations, which can feel a bit like winning the lottery.
What surprised me was discovering some university-affiliated short story contests for 'emerging writers' that offer upwards of $2,500. They aren't advertised on mainstream writing sites, you have to dig through university English department pages. The catch is they often require the work to be unpublished anywhere, even on a personal blog, which is a huge commitment for a single piece. It makes you weigh whether locking a story away for a year for a chance at a prize is better than just publishing it serially online and building an audience. Still, for a clean, unpublished manuscript, those can be a decent shot.