Silicone and glass, but also velvet cote plastic. Plastic is still fairly safe and sometimes I prefer something vibrating with no give (still not many glass vibes out there that I like), and I really like the velvet cote feel and matte appearance.
I prefer glass because of its higher qualities. It's durable, and easy to clean. Pyrex is my favorite because it has been said to break into larger pieces for easier and safer cleanup if it happens to break.