For one, a good backstory for why they became a villain. I hate villains that are evil for the simple sake of being evil. They need to have an understandable, but sympathetic reason, except special cases like Joker(who in the comics is at least Batman's intellectual equal). This is the only problem I have with Palpatine, no real reason behind his madness.
They need to be irredeemable as well, although they may not have once been evil, they are far past the point of no-return.
They need to be efficient. I hate how in videogames incompetant morons like Bowser and Sephiroth are given so much credit as a villain, but a competant and intelligent villain like Ganon does not.
They need to be badass, I mean seriously, everyone loves a badass. 😄
A good villain does not have simple mindless goals like "control the world", if their goal is world domination, the goal should be made more complex than that, like Ganon.
They need to look cool. Seriously, villains like Kuja are good...But the dude looks like a chick in a thong. That kinda stuff just bothers me. I personally like the big and muscular looking villains. Bit of a sick fetish for me actually.