I know I posted before... but I have to add that Ghost Bills, as someone mentioned, is about the most visual bill through effect out there... and it is completely impromptu.
Also, I still maintain that...
Misled, is the best pen/pencil through effect due to the motion involved in it (no cover... and it can move in and out of the bill, and the spec can see where the pencil goes in... plus there is a great 'melting' phase to the trick). Also, this effect is completely examinable, without switching items.
Pen-throughs... well... many people talk about how visual these things are... but they are VERY limiting, if you think about it. I would go with Misled over a pen-through.
Dough! by Sankey... is a GREAT impromptu effect... if only performed once for a spectator... EVER.
Misleading Misled is also a good impromptu... does not require any cover, like Dough! (post-it note)... but I have found that I would rather use my Misled, than perform the impromptu versions that are slightly less visual and more confusing to the spectator (even when done right).
So... in short...
You should buy Ghost Bills no matter what... and Misled is one of the oldest _______-through Bill effects, but it is still the most visual... and most inspectable for the WOW! effect that it delivers.