Ruthenium is the coolest element! Ruthenium complexes are approachable inorganic syntheses for newcomers and provide significant challenges to keep experienced chemists busy. The beautiful colors accessible via unique coordination chemistry of Ru make it useful for photovoltaics and electrochemiluminescent devices. Not to mention its utility in organic syntheses as a catalyst. Lastly, heavy metals rock!
I have to say the coolest element is Titanium (Ti). It is very stable, corrosion-resistant, alloys with other metals and is superstrong for its weight. It is used to make lots of really useful products, such as glasses you can basically sit on with out breaking the frame. It is lightweight also making it useful for the production of everyday things like medical implants and bicycles. And most especially, it is found in catalyst residues of polymers AND is really easy to quantify by X-ray Fluorescence Spectroscopy. I could give a ton of other reasons but then I would just sound like a total nerd!
I'd say that Tungsten is the coolest element. It's also known as Wolfram, which sounds a lot more intimidating than many of the other elements. Also, with one of the highest melting points and densities, it is quite remarkable. And to top it off, it is versatile enough to be used by some bacteria. I will admit that I am slightly partial to it since Tungsten's atomic number is the same as my college football number.