White Peony, as far as I know, is as bold as it gets, although I've
tasted some Sow Mee (shou mei?) tea that could compete with it for
strength. The latter tends to be cheaper, too. I like brewing the peony
at higher temperatures to make it stronger and more malty.

There also innumerable types of Assam and Green teas that you might
like. Temple of Heaven is a gunpowder, I think, and there are some
other strong greens out there. I couldn't recommend any, as I prefer my
green teas a little more subtle. Perhaps Bi Lo Chun (pi lo chun), which
has a strong distinct flavor, might be good to try.

In general, I suggest getting a number of small samples offered on
vendor sites. If you're in Los Angeles, I always recommend Wing Hop
Fung, because you can buy loose leaf tea quantities by dollar amounts
($2 of that tea, please!).

Anyway, best of luck, and post a follow up when you find something good!