Sorry, I was a little ambiguous in my metal references--I was trying not to make a monster post. (Like this will become)
No metal is a great heat retainer. Think back to your middle school science lessons. Metals are conductors; rubbers, woods, and ceramics are insulators (and a bajillion other things, but moving on).
Some metals, however, do hold heat better than others. For instance, copper is a quickie, which is why it's often used for wiring and in some higher end cookware. It heats rapidly and evenly, then cools rapidly. Metals that have a complex rather than compact/dense matrix, though, can also have some insulative properties. Cast iron, for instance, often contains 2-4% of carbon or sillicon and other impurities in its matrix, which helps it hold heat a bit longer than most metals.
I'm no metallurgist (basically trying to rembember as much of my undergrad chemistry as possible here), but from what I read it seems that silver is sort of similar to cast iron in this respect. It often contains enough impurities to introduce heat-keeping gaps into the overall molecular matrix. The other metals are easier to purify, and it is very easy to control the alloy formulas for steels allowing for a comparitively wide range of steel grades.
Soooooo....of all the metals you've listed, I believe that silver would in fact hold heat the longest, but this particular hypothesis will go untested by myself.
Of all the metals I've listed, I think cast iron would win the crown.
Porcelain is an interesting case. It's a pretty darn good insulator, but because the clay is so pure, it can be crafted into much thinner and more delicate pieces. A thin walled pot will keep your tea warm longer than you might think...but not above an hour (if that). The large bone china and porcelain pots are meant to serve a large number of people, making their insulation time a rather moot point.
If you want to be a gracious host...go for a large silver or porcelain pot. If you want to sip tea while you study or talk into the wee hours...you might want to check out some more "everyday" teaware. The ugliest Brown Betty will keep tea warm longer than the most delicate porcelain. Lots of pots even have warmers, like Adagio's Concert Teapot
Finally, metal cups burn lips. Your lips are the most sensitve skin surface on your body, so something you can hold comfortably could even burn your lips. You *can* find sterling silver teacup-looking cups, but these are actually punch cups, and punches tend to be icy. You'd be better off with a silver pot and a porcelain cup.