I have a few in mind.
Both of Ivy's unblockables: They are quiet effective and I can't tell you how many times I got hit by these because I happened to be too close to back away and too far to even hit her out of it. I can't remember the inputs, but these are probably some of the best in the game.
Viola's 44A+B with and without orb: It's all about the hip shakes baby~. With the orb it is pretty decent to throw out against inexperienced players or a semi-experienced player who happens to be stuck in the corner and holding block. I believe it can also catch regular step if they were not expecting it at first and were planning on blocking a jumping A+B at the beginning of a fight. Without the orb also has it's uses as it acts as a nearly unavoidable attack during her CE. Can be avoided with Just Guard during a raw CE.
Yoshi's suicide unblockables: Always a good laugh when someone accidentally uses these and kills themselves. More for laughs.
Cervantes' CE: Personally, I think this is the best in my list for a few reasons. It is ranged, kills noobs who can't duck, can be combo'd into, and can be stacked twice during a combo. It also has pretty deadly damage if I recall, which is not surprising for an unblockable. Downside is, it can be ducked and quick stepped.