Other. Titanium......just kidding
Short answer: steel, specifically butted, 4130 CroMoly. Steel is real and generally holds up together/better as a donor frame for motorizing. (That said, there are and I do own some high quality 7000 series aluminum frame MBs)
It can be useful to remember we are using frames, generally, that were almost exclusively and originally designed to be
bicycles. (unless someone is building proprietary frames, of course) I say that because steel bicycle frames evolved to aluminum bicycle frames with the idea they would be strong enough for
that purpose and the main advantage was to save weight.
Also, there were different grades of steel used in bicycle frame building over the years just as there are different grades of aluminum. A couple very pertinent components of this equation comes down to price IE: cheap vs good, and manufacturers origin. For example, a cheap aluminum frame welded in China is nowhere near the same quality as one made in Taiwan and neither as good as one made in the US. Of course, expect to pay (a lot) more each step up that ladder.
This is why I generally prefer and suggest, particularly if costs are an issue, it far better to look for a used or yard sale CroMoly steel frame bike, preferably of certain vintages, over buying almost any new bike at a big box department store..........better material and importantly, far better welds.
BTW, it is a good idea to treat steel frame interiors with readily concocted homemade rust inhibitors, something easily accomplished during the build phase.