If given the option, which recent US presidential election would you flip?

Lopen, your logic is accurate in theory, but not in practice. In theory, yes, it would be easier to push fraudulent votes in a pure popular vote, because there would be less oversight possible due to the sheer scale. And in theory, it would be harder to push fraudulent votes in individual states because they'd be under much heavier scrutiny. In practice, it's much different though.

The sheer scale of popular vote makes it so even small percentage margins are MASSIVE in terms of sheer numerical value. In the modern era of presidential elections, there have only been three elections (out of sixteen total, so less than twenty percent) in which the popular vote margin was less than 1 million. We've reached a point where the number of votes cast overall is so large that in order to rig the system and vote stuff in a meaningful way, you'd need to be incredibly blatant about it. And that level of blatancy, combined with a likely extreme divergence from exit polling data, would make it easier to ferret out attempts at fraud.

Meanwhile, because the state level elections are so much smaller, the margins of victory are smaller as well, which means it would take much less effort to try and push fraudulent votes in the right states to achieve a specific outcome. And while again, in theory, there would be more oversight to catch that fraudulent voting, the fact that elections are handled on a state-by-state basis rather than fully federally means that oversight depends entirely on the existing state government. And in situation in which one party is willing to engage in voter fraud widespread enough to swing a state from one party to the other, you have to consider the likelihood that they would have a back-up plan to avoid getting caught...or make it so getting caught wouldn't matter. All it would take is ONE right person in ONE right place (a statewide election official of some kind) to completely swing things.