It's not a simple answer. Alaska Airlines is one to consider, but it depends which airline you're actually going to fly, because Alaska airlines has different award charts for every partner, and with some partners you get great value and other partners not as great a value.

And unless something's changed recently, it may depend also on where you're starting from, because Alaska hasn't let people mix partners on one award, they only have allowed mixing one partner with Alaska Airlines metal flights. So if you don't live somewhere where the partner flight originates, and you don't live somewhere Alaska serves with their own planes, it could be tricky.

You can't transfer any other transferable points to Alaska other than Diners Club Club Rewards, plus one or two hotel programs including Marriott Bonvoy (but it's nowhere near as good a transfer ratio as from Diners Club to Alaska Airlines.) People who don't know about Diners Club often claim that Alaska Airlines miles are the hardest to obtain, especially since Alaska is known for cancelling accounts of some people who earned Alaska Airlines miles no other way other than churning multiple Alaska Airlines credit cards issued by Bank of America.