Delta now vs the other 2 legacies (United & American) is a big gap. American pre Bankruptcy & US Air merger was the Golden standard. But after they have struggled more the DL and UA. Just look at their stock prices vs the other 2. This despite being the largest airline in the world by multiple metrics.
Speaking of smaller carriers. Southwest whom were the customer service standard here in the U.S. in the 80s-00s, lost its mystique. They are overpriced and no longer a low cost carrier. I like 1st class too much to ever fly them anymore. But they still are the only carrier that still has free bags without holding their credit card. Virgin America before Alaska Airlines acquisition, was my favorite airline and goto when traveling domestic
Amongst the international carriers, nobody compares to Singapore Airlines, that gap is wide. Emirates for what it is is decent but your chancing it being hot boxed with a bunch of Indians transitingthru Dubai. I'd say Singapore, Qatar and Japan Airlines are the top 3 right now in the world