The honor of the worst seems to always fall onto Allegiant, Spirit or Frontier. Per Trip Savyy, here are their findings:
At the very bottom of the domestic airline rankings are Spirit Airlines (score: 2.5) and Allegiant Air (2.8), both of which are budget carriers. But the third worst U.S. airline is one of the big three: American Airlines. Bounce gave the airline a pretty low score of 4.2; it’s still vastly better than Spirit and Allegiant, but far below the top domestic scorer, Delta Air Lines, which earned an 8.9.
However, I have noticed lately that Allegiant has improved. They bought or leased newer planes and seem to be more reliable in terms of their schedule. I flat out refuse to fly Spirit and Frontier.
Delta, JetBlue and Southworst are the best rated airlines in the U.S. I have not flown on Delta for at least a decade, as its best routes are in the east and south. JetBlue has improved, while Southworst has regressed. Alaska is my favorite domestic airline, both for comfort, reliability, and on time performance.
Internationally, I tend to avoid the small airlines. Trip Savvy says the following:

Then on the international side, we have Viva Air Colombia, which scored just one out of five in each of the judged categories for an index score of 3.4. The best thing the airline has going for it? It somehow had the second-fewest complaints of all international airlines between January and June 2021, per the U.S. Department of Transportation.
Following Viva Air Colombia is Mexico’s Viva Aerobus (3.6), Volaris Airlines (4.0), and Ireland’s Ryanair (4.2), best known for its backpacker-friendly fares across Europe.

Out of all of these, I have only flown Ryanair, which I find acceptable, and reliable. In Asia, AirAsia dominates the budget airways, and it seems also to have improved over the last few years. Easyjet is another budget European carrier with scheduling issues.
What about the best airlines?On the other end of the spectrum, the world’s best airlines are a bevy of Asian carriers, including Ana All Nippon Airways (9.6) and Singapore Airlines (9.5).
Singapore Airlines is my “hands down” favorite airline. Their Business and First Class are superb and cannot be matched by the big US carriers. Their lounge is also the best! My next trip to Japan will be on either Singapore or ANA.
But always remember two important facts. One, the urgency of getting from Point A to Point B. And two, the price, if that matters to you. You are the only one who can decide how to juggle these two reasons for flying.