Looking for recommendations for a PC based GM specific scan tool

I am looking for a fairly comprehensive GM specific PC based Scan tool package for OBD 2. For use on a 2011 Silverado, 2004 Blazer and 99 Pontiac Bonnieville (not as important as the 2011, the others have
over 150K miles and will be gone in a few years). All opinions welcome. Thank you.
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