Better is an entirely subjective value. The one you think is better is better. That being said, here are some guidelines. Trout, including steelhead, are usually milder than salmon. The richness in flavor is directly correlated to lipid, that is fat, content. Another fairly objective measure is flesh firmness, which is related to water temperature that the fish lived in.
The fish with the highest lipid content are the ones with the longest spawning migrations and the longest amount of time from river entry to actual spawning. This favors spring chinook from the upper Columbia and Snake Rivers and the early Stuart run of Fraser River sockeye.
Cold water favors the Copper River and other Cook Inlet area sockeyes (reds) and kings. The length of migration and time until spawning of these fish is relatively modest, but I find them to be absolutely prime table fare.
Beyond that, there are other variables that affect fish flavor and texture, like how fresh (we are talking fresh fish only here, of course!), how the fish has been handled since being caught, and how well it is prepared.
With that in mind, I rate spring and summer kings tops, sockeye second. After that, comes everything else, in no particular order for me. Although I rate silvers and steelhead above pinks. And chums, bright ones that is, smoke well, and they were OK the one time I had them barbequed. And for what it's worth, I'd rate farmed Atlantic salmon just below steelhead and resident rainbow trout. And I dearly miss the extremely delicate flavor and flaky texture of harvest trout. Alas, I release them all anymore.
And just to remind you how completely objective my evaluation is, I should mention that I make absolutely, to die for, the best barbequed salmon in the world. It must be; no one has ever indicated they've had better.
Oh, and I agree with a couple of the others that halibut and ling cod are mighty fine fare as well. And an interesting thing about halibut is that it freezes better than any other frozen fish I've had.
On that note, only four months until the first Copper River fish arrive!
Sincerely, with a little t.i.c.,
Salmo g.