Best slip on shoes for flying?   

Slip on shoes are fine unless you crash and catch fire and lose one or both of your shoes and have to exit through burning jet fuel. Never wear any shoe that can melt on your feet while you are trying to extridite yourself from a crumpled cockpit. I used to see a lot of Robbie pilots flying in shorts and tennis shoes and you can see a lot of small airplane pilots doing the same. If you watch enough warbird pilots they wear a nomex flight suit, nomex gloves, a helmet, and tennis shoes that will melt on their feet. At a lot of airshows the "fire guard" for those large radial engine planes are wearing a T shirt, shorts, and tennis shoes. On the 737 I'm on now I wear black leather "penny loafers" as I figure any crash will kill me anyway. Your milage may vary, but at least wear a leather shoe.