I use Quaker Old Fashioned Oatmeal (or a generic version). The more processed it is, generally the higher the GI, so most advise staying away from the instant stuff, especially the packets mixed w/ sugar and junk. Steel cut oats are supposed to be the best, but they're hard to find for a decent price in many areas.
I just toss the oats in a blender w/ milk, whey, fruit, etc. It's a little gritty especially at the bottom of the shake, but that's no big deal to me.
Nobody's saying you NEED to use oats, but they're probably the best carb choice to add to a shake.