After doing some research about the best scan tool to get for my GM cars, it really boiled down to what the factory service folks use, and in the era of my cars, that is the GM Tech2 scan tool. They aren't made new anymore, but you can get knockoffs, likely built in the same plant as the originals, as new for a decent price, so that's what I did. Here is the unboxing of this tool.

Purchased on eBay from a seller no longer selling them.