Yup.. it totally depends on what you are making and your preferences. Sorry that's not much help

For a general elastic that is easy to get, I like a 1/4 inch knit elastic. You can get it at walmart.. it's by stretchrite. You can also get polybraid, but I feel like the return on it is too strong and it leaves marks for my dd (but some patterns like the VB pattern call for polybraid.. so it's totally a preference thing) . If you are ordering online, a really nice elastic that has a lot of stretch but the return on it isn't so strong but stronger than a knit elastic, is swimsuit elastic. I get mine from diapershop.com
You'll eventually figure out what works for you, so don't buy too much at once of any single kind. But overall, I'd say 1/4 inch wide elastic. Try some knit, some polybraid, and some swimsuit elastic if you can. Just get a couple of yards each (most diapers can be made with about 18 inches of elastic give or take so two yards will get you at least three diapers in most cases) so you can give each one a "stretch test" and see what you like with the pattern you are using.
There are others too, but those are the easiest to work with and find so stick with those for now.
Amber