What's the best material for a durable muffin pan I make two or three dozen muffins a day for various customers. I don't like to use disposable cups, so I grease the pan each time. No matter what I do, after a certain time, my muffin pans end up giving a subtle metal taste to my muffins. 

I was wondering what would be the most appropriate material for a muffin mold to avoid this situation. I'd be ready to pay the price for a pan that would last significantly longer... Bug which one should I choose?
First, I concur with @rumtscho that the pans may not be the culprit and second, I do not make muffins in anywhere near the quantities you report, but I have had great luck with silicon muffin trays. They clean well and release the muffins nicely.