Often the pattern will tell you what kind of stitches to knit in the gauge swatch. Many times it's stockinette (even for some items that predominantly use a pattern).
Since you're trying to figure out how large the finished project will be (without making the entire thing first), it makes the most sense to do a swatch (or swatches) in the stitches (or patterns) that are used most in the project. (So if you're making something that's predominantly garter stitch with ribbing, you would do your swatch in garter stitch.)
Note that cables will pull the knitted fabric together and lace will stretch it out.
The only time I do a swatch for ribbing, by the way, is when I'm trying out a new type of ribbing. Since it can be either very stretchy or not very stretchy (oh, well, and everything in between, too), it's a good idea to test out how the new ribbing behaves. However, if I'm using k1, p1 or k2, p2 ribbing, I don't worry about creating a swatch for them.
Creating gauge swatches is a great idea, especially if you can create a log of information about the yarns and needles you commonly use. I have started using Ravelry's yarn stash feature to record this kind of information. Then when I want to start a new project, I can go there and see what gauge I got for that yarn last time. I still may need to adjust needle sizes, but at least I have a starting point.