Like a lot of frequent UA customers, I'm concerned about the future of my miles. Any thoughts on which *A program is best to deposit mileage from UA flights?

UA MP is your best option by far, especially when you frequently fly on UA metal.

US DM comes in second, but that airline is in even greater peril than UA (that might change when the merger between UA and HP works out).

However, I can't identify any serious risk for UA miles at this time. UA is big enough and such an important part of *A that even in the highly unlikely case of its liquidation, another airline would step in and comp your status/miles.

But as *A simply cannot function w/o a big North American member, so UA will just be kept alive, whatever it takes. As many of us will know, UA's CEO recently urged American legislators to lift restrictions regulating foreign ownership of U.S. airlines. He obviously did that for a reason.

Based on what little info you gave us on your travel, I'd agree with flysurfer above that MP may still be the best bet for you. I also agree that should UA liquidate, there will be status matches galore to be had (I'm not so confident about matches of mileage balances, though).

If you are very concerned about your miles, consider burning some of them.

With airlines in peril and programs in flux (look at what's been happening in the european programs over the last year plus), I've moved away from hoarding to burning... Just figure out what you'd want to do with your miles and keep your account at that level by balancing your earning/burning. With UA, for example, I like to have enough miles for two *A awards in F anywhere in the world. When my balance starts getting higher than that, I burn miles on upgrades, domestic trips for family/friends, expensive intra-European flights, etc..