510 Pro DCTank Combo
I have several of these in all the colors and both sizes. The tank casing is made of polyprolyne, the same stuff medical syringes are made of, and are totally safe for use with vicious e-liquids like cinnamon and fruit flavors. They are not clear, but frosted, but you can still easily see your juice levels. They are actually made tougher than the usual clear polycarbonate tanks. I started out with a couple of those and they both ended up leaking, and not because of the juice I was using. If you decide to go with the polyprolyne tanks, if not specifically listed as such look for the frosted appearance.
Of course, pyrex glass is the best, but I couldn't afford a Phiniac at the time I got my polyprolyne tanks, as I needed several. I put color-matched Ming and Trippy Tips on each one to help tell my juices apart. I will eventually own one or two, tho.