I would not pick what to study based in a vacuum on what has the best ROI. If you do that, you might pick something you don't particularly care for or aren't particularly good at. That would likely lead to a less-than-expected ROI and lower than expected happiness.
Instead, I'd do some soul-searching about what you truly want to do, and determine the best way to go about getting it, which might include more education.
I got my MBA after working for 8 years and realizing the specific career change I desired. It focused my studies and job search well and I got what I needed and wanted. Other classmates that were enrolled because of the high perceived ROI or because of a vague sense of wanting a change, lacked the focus and ended up meandering a bit, being attracted to the glamorous fields others were chasing, and have bounced around in the post-school workplace figuring out what they really want to do. Some are doing things they love that absolutely did not require an MBA, which means they wasted a boatload of time and money.
That said, if I got free tuition and had the time, I'd pursue a PhD in economics just for the fun of it. I LOVE economics.