In general, the darker the color of the metal of the muffin tin the more it will brown its contents. You also want to look for a thick, durable metal as this will aid in even cooking. Finally, look for a tin with large, wide handles; the last thing you want to be doing is sticking your thumbs into a muffin when trying to insert/remove the tin from the oven! Most muffin tins these days are non-stick, so it may be hard to find one that is not. With that said, non-stick technology has advanced a bit since the Teflon-only days. For example, many modern non-stick pans/tins instead use other metals and ceramics that are not known to be harmful. Furthermore, some modern muffin tins are marketed as "metal-safe", meaning that one can use a metal utensil on them (e.g., to extract the muffins from the tin) without damaging the non-stick coating. Finally, muffin tins will likely never be heated to the temperatures at which the "dangerous" non-stick coatings release harmful gasses.