I'm actually not sure myself ^^ can't beat argon xD I mostly keep a list of favorite organic chemistry solvents I've used over the past months lmfao. DMSO is my favorite by far there, btw. Not overly dangerous but dissolves most stuff. Ah.
Perhaps Natrium or some other metal you can cleave like butter. So cozy looking and waah bright lights in water, yay!
EDIT: oh right, it's Sodium in English, right?