There's really only two criteria.
Most would say most popular sport is a mix of the two. Each would have their own weighting though.
1) Raw population. How many people watch? How many people play? How many man hours are spent training for or playing that sport?
2) Money. What are the league's (or combined individual) revenues?
By criteria 1, what most would consider a niche sport like cricket enters discussion. I think even cricket fans would find it silly to call their sport among the most popular.
By criteria 2, golf enters the picture - you kind of need to be (relatively) rich to play golf, and need access to very expensive to build golf courses. This is largely why golfers are not the most elite athletes; they are drawn from a much smaller population. But the average golfer spends vastly more money on his play than the average soccer player.
The NFL also enters the discussion because the total revenues of the NFL likely exceed that of pro tennis. It's US only, yet is so huge in the US that it has a good argument for more popular than tennis. It's also played by a huge population simply because teams have 22 starters each, multiplied by most high schools in the country and you get big quick.
Even unpopular NFL teams like the Buffalo Bills are worth a billion dollars. Vastly more than the number of billion dollar soccer teams.
Going by a mix of both criteria, soccer is say wins in most people's minds, even most Americans would admit this. What is second? Debatable. I'd say basketball.
NASCAR is considered a joke in most of the US so just forget that.
Boxing is up there certainly, though combat sports may have the biggest disparity between number of viewers vs. number who have ever competed in it. People watch combat sports, almost nobody ever does them. They also have no public school pipeline like most sports discussed (except for MMA and wrestling in recent times).
Remember when it comes to money, the US has a very outsized influence. Not only because it's the richest nation, it's also the most sport obsessed - as our Olympic medal counts indicate, mentioned earlier in this thread.
So if say
1) Soccer
2) Basketball...probably. Though before the Dream Team, it wouldn't be here.
3) ??? American football, Tennis, golf, baseball and others might be argued. Honestly it's probably American football going by money and total man hours played.