CO vs DL vs NW
 

I am wondering which ff program would be best for consolidating miles on for someone who just does an occasional flight on skyteam flights. I overwhelmingly fly *A, but I have a couple flights coming up on Alitalia and Alaska so I'd like a place to park those miles. I'm leaning towards CO after reading up on all three, but it's hard to determine from their which would be best. It's unlikely I would obtain anything more than a 25k mile status at best, so I'm wondering which would be best for an occasional award ticket and lower end status.

NW has more options for earning miles with partners to top off an account. DL has had a number of promotions that also included MQM earning with partners recently, but those are all gone now. NW/CO have reciprocol upgrade benefits. And then there are the partial credit issues where cheap fares on DL/NW earn partial on CO and cheap fares on CO earn partial on NW. I'd pick NW lacking any other motivating factor. Of course, the best bet is to earn on the carrier you fly the most.

CO's 50% EQMs policy hurts the occasional lower fare flyer, you have to try extra hard to make staus

NW is probably best of the 3 for your situation

NW is the best bet (of the US options) for most people looking to join Skyteam. I can see benefits of CO as an airline but I cannot think of any advantages to Nonepass...

Another thing to keep in mind is if you use SPG points to top off accounts, transfers to CO only give you half the number of miles as you would receive if you transferred to DL or NW. There is a way to transfer through Amtrak to CO and still receive a 1:1 ratio, but its more of a hassle and you are limited on the number of points you can transfer out of Amtrak per year unless you have status.
If you compare lounge systems, NW more international locations. Making Elite on NW would give you a discounted membership fee to a better lounge system than CO's.