Southwest is dollar-based redemption, so most useful if you can reserve a flight far enough ahead to get their 'Wanna Get Away' fare bucket. Once they sell out of those, redemption rates can be surprisingly awful, especially if your route involves connections.

My understanding of Hilton is that if you want the program to be genuinely useful, especially for higher level redemptions, it's best to jump in with both feet and max out elite bonuses and credit card multipliers.

I'm not familiar with Alaska, and Virgin's route map is limited so I'd probably just got for the American miles since they're quite versatile.