Aluminum for weight and price, generally. Stainless if you're going fancier, sterling silver if you're getting your swag on. Gold if you've just gotten an inheritance and don't know what to spend it on. Brass and copper are actually rather popular for their "rustic" or "faux-medical" appeal, bronze is a bit unique, you don't see it much, and that can work to your advantage or not. Your silver colors won't tarnish (aluminum, stainless, titanium) but your goldens will (copper, brass, bronze, etc). Except silver itself will also tarnish and turn. In the end, for casual wear, I find aluminum to be lightweight, affordable (don't call it cheap), hypoallergenic, and does not rust, tarnish, or turn. It's not just tarnish-resistant, it's just incapable of it! Plus anodized aluminum comes in all sorts of fun colors~ Rubber also has a definite appeal, it's easier to slip it on and off, and can fit more snugly, but some people find rubber to be "childish" or "cheap" (while it actually costs more than the metal itself!). A clasp tends to be "fancier" and will likely be the most expensive single part of the whole piece.
18g 1/4" is my favored size, it's solid and works well in a number of weaves. Some sort of 16g 3/8" is a bit thicker, masculine, and similarly thinner will come across more feminine. Size is your call, though large rings tend to draw attention to the rings themselves, and smaller, tighter rings draw attention to the weave and the style. If going for anodized aluminum or enameled copper, you can show off the colors by using larger rings, while a tighter AR will tend to make colors "blur together" just a bit - for good or for bad. You can accommodate small sized color rings by making larger color swathes. If doing stripes, make them 5 wide instead of 3, using smaller rings, for instance. If you're just going for generic aluminum or copper, then smaller rings can give you a tighter style, showing off the intricacy and the "bigger picture" as it were. It can be particularly fun, to use tiny rings to make a traditional weave. While a larger size might get 3-5 rows in, and look like interlocked chain links, a smaller size can get like 11-13 rows and really show off "proper chainmail" like you'd expect to see on a shirt, while still being small enough to wear casually, if that makes sense.
Also if you don't feel like making jewelry, you can always send business my way. *cough* Girlinhat42@yahoo.com