extremophiles - What is the most heat-tolerant organism?

I'd add that biochemically speaking, animals and microorganisms from thermal vents tend to have a higher GC content in their genomes, which makes it more thermostable.

All of the proteins are adapted to work at higher temperature. There are a higher number of salt bridges and specific hydrogen bonds in the proteins to stabilize them - more arginines, more glutamic acids, etc. While this reference says that Cysteine is less common, I imagine anything which is not in a reducing environment will have more disulfide bonds - e.g. for proteins on the cell surface.

Sulferous deep sea vents would tend to be reducing as is the inside of the cell, but maybe in the case of fresh hot water spring extemophiles this may be the case.

Reference:
http://peds.oxfordjournals.org/content/13/3/179.full