Hmm, well, seeing as Palpatine is my avatar, and he's definitely one of my favorites, I should probably give some thought as to why...
1.) He wears a black robe. You have to admit, he makes that simple black garb look stylish, even with his ghoulish face and reptilian yellow eyes.
2.) In the Original Trilogy, he makes a brief appearance in The Empire Strikes Back, just enough to let everybody know that Darth Vader has a master and that he's only number two in the big bad Empire. Yes, there's someone even more evil and cooler than the big guy in wicked black armor sporting a black cape and killing people with invisible choke holds.
3.) When he makes his big appearance in Return of the Jedi (my favorite Star Wars movie) he shows his stuff by whipping out some ultra cool Force Lightning. He slams Luke with sharp come backs in their dialog and manipulates him on a mental level, showing his superiority as the ultra smart, nefarious villain. Not only that, but he breaks out a trap for the Rebellion Fleet, nearly wiping them out.
4.) As far as death's go, his was pretty good. He went out fighting and was killed by his number two. Plus when he died, he exploded in some weird spirit explosion, showing just how evil he was. Palps was bad to the core...literally
5.) Don't forget the awesome bodyguards he had. Those Royal Bodyguards in their shiny red armor, armed with those weird looking Force pikes looked totally cool and kept their mysterious edge, even for just being eye candy.
6.) With the prequels, we get Palpatine's Darth name, Sidious, which is a pretty good Sith moniker. It's kind of like hideous, but cooler. And even as a plain old politician, he's still kind of cool because we all know that he's got this deep dark secret and is completely duping everyone else, even Yoda and Mace.
7.) Palpatine produces some great apprentices: Darth Maul, Darth Tyranus, and of course, Darth Vader.
8.) Palpatine has one of the most stylish lightsabers. It's kriffing golden. The only other lightsaber that even competes in his league of style is Mace, who also sports a pimped out golden lightsaber.
9.) Palpatine gets two outstanding fights in the prequels: one against Mace, the other against Yoda. He starts off his duel with Mace by killing three Jedi Masters. Oh, and he did it in the blink of an eye. In my opinion, his duel with Mace only lasted so long because he was toying with him. He needed Anakin to show up so he could finish turning him to the dark side. He finishes the epic duel by killing Mace. With Yoda, he shows off his telekinetic Force powers by throwing the Senate at the little green frog. They sword fight, and then Yoda runs off like a little chicken, trembling in terror. That's right, I'm calling Yoda a coward. Ha ha
Go ahead, watch Revenge of the Sith again and tell me when Yoda is crawling through that air vent that he doesn't look scared. To me it looks like he's got the fear of death on his face. He knows he just got beat, heck, he knows the Jedi have totally lost. Right at that moment, he was thinking "Sent Obi-Wan to Mustafar I should have not. Together we should have been. Got my butt kicked I did. Fail epically I have."
10.) This last one is simple: Palps is the man because not only did he fulfill Bane's Order of Two by finally bringing down the Jedi Order, but he also took over the Galaxy as Emperor. Emperor of the Galaxy. Doesn't get much better than that.