Quote Originally Posted by Orangutango
Mine have plastic tags and I love them! The alluminum ones from Petsmart (or similar stores) seem to bend out of shape and the colour wears off easily. Stainless steel & brass tags are nice too, but I like the plastic because they're just as strong as those ones it seems (depending on where you get them from), they're quieter and they come in a huge variety of colours. I also find them clearer to read.
I've found the plastic ones can get chewed up easily.

I vote for stainless steel, OR the stainless steel tag that slides over the collar. The aluminum tags are just too flimsy, scratch/dent easily and wear off a lot easier over time.