I'd say there are three types of champions that get pentas.
Type one is champions with mobility skills(especially if there is a steroid or some form of crowd control built into it OR the champions has a low CD CC option) that refresh cool-down upon getting a kill, this class includes Yi, Akali, Eve, Katarina, Irelia and Tristana off the top of my head, these are the guys with the best chances of penta, starting with champs like Akali, Yi, Eve, Katarina and Tristana... Irelia is actually pretty dang hard to get a penta with compared to the rest of this crew because actually has to kill with her mobility move itself for the cd to refresh, but on the upside if she can one shot minions with it she can always get around that way and use her OTHER capabilities on enemies... Eve actually is pretty strong for pentas because she gets health regeneration and a refreshed massive move speed and attack speed boost ultimate any time she gets a kill.
Type two is the champions with very difficult to pull off but incredibly powerful moves (Nunu, Katarina, possibly Fiddlesticks and Karthus) that if they manage to get all their timing right in the gap where no one has CC available can do a ****ton of damage all in one go on a lot of people. As you can see, Katarina actually belongs to groups one and two... however she can pretty much be completely shut down by one halfway hard CC at the wrong time, so she only really has a better chance of a penta than other members of either group versus unorganized teams or poor players.
Group three is ANY AD carry late game. These guys shovel out such an insane amount of damage that they have a pretty good chance of getting a penta out of a teamfight (but only mid-late game) sooner or later, the especially nasty ones are the ones like Vayne, Ashe, Sivir, Tristana and Kog'Maw who are really really hard to escape from after a losing fight. Tristana is in groups one AND three WITH A STRONG SLOW ON HER REFRESHING MOBILITY MOVE, endgame built she can also put out a ****ton of damage with excellent mobility and a bit of aoe damage any time anything dies. To top it off she has quite possibly the most powerful positioning skills of any ranged AD carry and the second longest auto-attack range of any AD carry ever, only losing to Kog'Maw when he's using Bio-Arcane Barrage once she gets to level 18 because of her passive.
The three top champions I would call for getting pentas are Tristana, Katarina, and Akali. However, smart cooperative decently built teams are pretty much guaranteed to keep Katarina from doing anything, and 125 gold once or twice can practically destroy Akali's laning phase if done early, so I would actually say that the strongest of these is Tristana, due to there not being any easy way to shut her down, she even has a built in healing reduction effect to make sure that Soraka/Sona/Alistar/Taric/Karma/Janna/Sion aren't saving her target easily.