My favorite element is Plutonium. It has an infamous history of course but also has some of the oddest properties such as changing crystal structure with temperature and entering criticality. The naming of plutonium is also fascinating as it logically followed uranium and neptunium. With the demotion of Pluto from planet to dwarf planet, plutonium joins cerium as the only elements named after dwarf planets!
Maybe my work has swayed me, but I'm a fan of sulfur. Versatile redox properties, nucleophilic champion, a mushier cousin of oxygen. You learn to not mind the smell.
My favorite element by far is copper. As a metal is has so many uses, great conductor of heat and electricity, relatively non-reactive, and a staple metal in construction and engineering. The metal also has great physical qualities. It's the only non silver-gray metal besides gold in its pure form, and its color when freshly cut is a pinkish orange that is so lustrous, and even the slightly browned orange after exposed to air for a while looks very nice. And of course if left even longer, it turns a greenish blue famous in the statue of liberty. In addition, it has a wonderful blue color in solution, especially the amine complex, and crystallized copper sulfate is quite a great sight. This is why copper is, hands down, my favorite element.
Got to say tungsten. While it may be a rather simple element compared to some, its strength and durabiliy make it a very useful element. It's alloys make many things possible. And lets not forget, without it you'd probably still be doing your reading by candlelight.
I would have to say Potassium. It plays such an important role in cellular chemistry and communications. There are lots of new discoveries being made about how ions work in the living system but we don't normally worry about getting enough in our foods as much as other vitamins and minerals.
Not necessarily for any of its properties but more for the memory it brings back. It reminds me of the semester I took Advanced Analytical with a solid group of friends from undergrad. Our instructor did MS work involving a collision cell. Many of his examples involved Argon, which became a running joke in the class. It reminds me of good times and the good bonds (pun intended) formed within the group.
My favorite elements are lithium and carbon since they represent the first two letters of my first name and the the first letter of my last name. Lithium is also cool because of all of the applications it is used for including batteries. Carbon is my other favorite because it is the basis of all life!!!