Others who know the ins and outs of spg and the various airline programs might know better, but my guess is that your best best is AA.

Unfortunately, unlike most airlines to which you can transfer spg points, UA and CO only offer 2:1 rates. (E.g., if you have 10,000 spg points you get only 5,000 miles.) Most other airlines offer 1:1. Another option within the Star Alliance is US, but it's a horrible airline and has the worst prospects for surviving, so you should steer clear of it. Lufthansa is yet another option, especially since you'll be going to FRA. But you can't get a credit card (and thus accrue miles for spending) for that airline, plus I'm pretty sure if you're flying economy (which I assume is the case, though you haven't said) you won't accrue many miles for your PIT-FRA trips, for reasons I detail below.

Which brings me to a related point. You're not clear in your post, but if I understand it correctly you aim to use your PIT-FRA trips to accrue rather than redeem miles. It might be a slightly different calculation of airline priorities if you aim to redeem miles to fly there. But in either event, you still bump up against the problem of the 2:1 transfer to UA and CO that I mentioned.

And, assuming that you see PIT-FRA as a means of accruing miles, be aware that, like Lufthansa, some airlines don't give full miles for low-price economy tickets. That is, if you buy the cheapest economy ticket to go PIT-FRA and assuming that route is about 7,000 miles round-trip, you might only accrue 3,500 miles or less.
I forget which are the better or worse airlines in this regard--I'm not sure where AA stands.

You didn't mention Delta, which is good. It's program is notoriously lousy for trying to redeem points. So you could pile up points with Delta and not be able to use them, or find that you have to use much more of them for free travel than you would with AA, UA, etc.

So, by default, I think you're at AA. But perhaps others can shed some more light on some of the matters I raised here.

By the way, here's the link for the spg page that provides the transfer rates from spg point to airline miles. Scroll down to "Transfer points to..." and then under the airline miles heading you can click to bring up the pop-up that lists the airlines and their transfer rates.

http://www.starwoodhotels.com/prefer...ers/index.html

Hope this helps!