I would have to say that platinum is my favorite element. My first major paper was on the synthesis of cis-platin and other similar anti-cancer drugs. Part of my doctoral study was the preparation of another platinum compound.
My favorite element has to be hydrogen - atomic number 1. The first element produced in our universe. Our sun fuses hydrogen into helium and shines onto our planet to support life. Hydrogen combustion holds the promise of non-polluting energy (the only product being fresh water, which is in short supply). Ironically, hydrogen generation is often a foe that limits my electrochemical work in aqueous solutions. But that's OK. Hydrogen has already done enough for me, so I'll let that one slide...
My favorite element is number 72 or hafnium. Among other things it was the cause of my meeting Sir Karl Popper after I wrote to him about what he had said on the subject. Popper famously claimed that the discovery of hafnium, following Bohr's prediction, represented the most dramatic case of the reduction of chemistry that he was aware of. Here is a short piece I wrote on Hf for a Royal Society website,
The element is number 72 in the periodic table, and is now called hafnium. It takes its name from hafnium, the old Latin name for Copenhagen which is the city where it was first isolated in 1922. But first let me back-track a little. In 1913, the physicist Henry Moseley, working in Manchester and later Oxford, discovered an experimental method for ordering the elements according to their atomic numbers. Prior to this work the elements in the periodic table had been ordered by using their atomic weights, which gave rise to a series with uneven gaps between each element. As a result, nobody could be sure how many elements remained to be discovered. All this changed following Moseley’s discovery because atomic number increases in whole number steps as one moves through the periodic table.
One of the gaps that opened up, was between element 71, lutetium, and element 73, tantalum. Moreover this particular case was complicated by the fact that it was not clear if element 72 would turn out to be a transition metal, or perhaps a rare earth element, since element 72 falls at the boundary between these two types of elements. Some chemists thought the element would be a rare earth element and carried out many fruitless searches for the element among minerals containing rare earths. But some other chemists suggested that the new element would be a transition metal. The chemical argument for this was quite simple. According to some versions of the periodic table, element 72 fell underneath titanium and zirconium in the periodic table, and both of these elements were known transition elements. Then an argument from physics was proposed by Niels Bohr, one of the founders of quantum theory. According to the electronic configuration that Bohr predicted for element 72 he also agreed that it had be a transition metal.
In 1923 Coster and Hevesy a couple of young researchers in Bohr’s institute decided to try to isolate the element as a test of Bohr’s theory. In order to do this they followed the chemists’ suggestion and decided to look among the ores of zirconium. Within just a few weeks they succeeded by examining some Norwegian zircon and by detecting the X-ray spectral line frquencies expected for this element. It was the discovery of one of the only six then remaining gaps in the periodic table. It also turned out to be the one but last discovery of any naturally occurring element, the last one being rhenium a few years later.
Hafnium is not all that uncommon compared to many other exotic elements. It occurs to the extent of 5.8 ppm of the Earth's upper crust by weight. The reason why it took a long time to isolate is that its atoms have almost the identical size to those of zirconium, along with which it typically occurs in minerals. This makes it difficult to separate from zirconium. But these days a number of methods of extraction have been developed and hafnium has found many of applications because of its rather specific properties. It is a shiny, silvery metal that is corrosion resistant to a remarkable degree.