There are quite a few good "tuners" in town.
The issue is - how much do you want to pay and what do you expect? Canned tunes are ok for something like a mostly stock crowd - but they will never beat out an actual tuning session in person. The reason canned tunes are so popular is the price. If you go with a canned tune - make sure they do revisions, where you send them datalogs and they tweak it for your vehicle. The only "canned" tune I ever used was from Torrie @ Unleashed Tuning - and he made 3 or 4 revision via the datalogs on my SHO. I don't know if he does GM stuff - but if he does Unleashed Tuning is worth looking into as I was happy with the whole process.
In town - there are plenty of reputable tuners. I have personally used a few depending on what I was having tuned - Rich and the crew @ Dynocomp, Tony @ UMS, Mark @ SpeedSports.