As an airline I personally like Alaska and Delta as my two favorite US based major airlines (if you consider Alaska Major). Personally I find Alaska's program the easiest to earn with and the easiest to find good redemptions on. That being said there have been some signs with new partners they have added that we could be headed for a devaluation in the award chart, but that can happen anywhere. My Pro/Con List for Alaska:

Pros

• Best Mileage Earn and Use program in the US (Really good partner earning too)
• Great Service
• Decent Food (probably better than AA in my experience)
• One World, so massive partner list
• Lounge Access for Domestic Flights if you are in First(and their newer lounges are really nice)
• Reciprocal upgrades with AA for 75k and 100k MVPs
• Great West Coast Availability,

Cons

• More Expensive (in my experience) than AA/UA, seems to be on par with DL (I fly out of Sea so that could be why it is more expensive)
• Only Narrow Body Planes (So no real opportunity for lie flat on a TransCon flight, unless you are 75k or 100k and flying on AA, but again you only get one class of Service upgraded, so if there is premium economy you will not be bumped to business)
• No seat back IFE (This may not bother some people, but especially on TransCons I like IFE)
• Reciprocal Upgrades with AA. This is a pro and a con because it will likely result in longer waitlists and making it tougher to clear (on both sides)
• Limited East Coast Availability (This can also become an issue when flying in the winter when we are talking about rescheduling/cancellations)

Generally I split my loyalty between Delta/Alaska. They are by far my two favorite *Edit* DOMESTIC airlines. I do Delta more often for longhaul and Alaska for my West Coast Travelling. This results in me having low tier status on both.