Thinking about it, I'm pretty sure glass takes longer to break down and degrade than metal, so that'd be my guess.

Unless you mean toys under proper care and not left to the elements, and assuming durability is also to be taken into account, in which case glass would still last an impressively long time, but steel would be harder to break (though the glass in toys is pretty strong). Silicone can also outlast its owner if cared for properly.

In terms of toy materials. Glass and metal are probably the same.

In terms of all the elements. I believe gold is the most stable. It does not rust (oxidize) and it is incredibly inert. All of the noble gasses also do not break down, but those are gasses.

Ceramic or glass toys would last the longest. As long as one was careful not to drop or scratch them, the materials they are made of will last thousands of years.
Silicone is sensitive to light (so store away from light), but seems long lasting (from the quick search I did on Google).
Not sure about metal, probably depends on the type. Gold doesn't tarnish (that's why the Egyptians used it to make their stuff for the afterlife), but is very soft.