It's just about impossible to answer the question 'best' comprehensively. One can transfer SPG points to 30+ carriers. Beyond that, one can use those miles on programs which aren't directly SPG transfer partners (as with the idea of transferring SPG to Delta, and using Delta miles on Virgin Australia). The number of possible combinations just explodes.
That said, SPG to Alaska for travel on CX can be a good idea. AS allows a stopover - American Airlines is much more restrictive on international stopovers by comparison. There's no fuel surcharge on CX, unlike travel on BA (with BA, AA and AS miles, anyway). The AS partner desk has a good reputation for working hard to find available partner carrier routings.
Do make sure that award travel is available on Alaska (to get you to the CX gateway city) and CX in both directions before transferring SPG points to AS. You can't undo an SPG transfer to AS. You can't mix partner carriers (except AF/KL) when redeeming AS miles, as by flying CX to BKK and flying Delta back. Delta has more permissive stopover and carrier mixing rules but low-band award inventory on Delta-operated flights can be tough to find.