The actual booking class is really important in your situation. You may find that A3 or OZ don't credit very well if you're forced to fly on the cheapest economy fares (for example L, T, K, G fares on UA), however if you're flying middle of the road fares like V or Q then you'll get solid credit almost anywhere.
wheretocredit.com is a good starting point. Typically the easiest programs to get *G are A3, OZ, and SK but it's not set in stone and depends on the actual fares you'd be booking.