What is the best metal for a tattoo machine?

It's all relative. They all have pros and cons. If you want a light machine aluminum might be for you, if you like a lot of weight in your hand go with brass. Iron is kind of the common inbetweener. But there are machines made of everything out there, including carbon fiber and wood! Irons are a good go to, brass are the best (in my opinion) if you can cope with the weight. Brass just absorbs vibrations the best. Hope I helped ya out. Also, remember just because you like brass or iron does not mean all brass or iron machines are good. Find a brand/builder you like and go from there. Most builders have a few different models consisting of different metals.