I feel amused whenever I see that idiom. So here you go - whats the greatest thing since sliced bread? ;) [I know Paleo diet is a good answer, but post something else!]
The phrase is a misnomer. Nothing has yet to top bacon. And it's likely nothing will.
And, in all seriousness for those who didn't already know and are interested in this kind of trivia, this phrase evolved from a marketing campaign:
So, I think it's imperative, particularly in light of the recent declaration of war on cupcakes (http://hunter-gatherer.com/blog/declaration-war-cupcakes), that we do our best to ensure that the phrase, "this is the greatest thing since bacon (but how can that be, as nothing is better than bacon?)" become commonplace and use it until it displaces that vile bread one.
Requisite emoticon, signifying the tongue-in-cheek nature of this answer -> :)
At this point, having just tried bone marrow for the first time (well, second and third followed pretty quickly since i had so many marrow bones), my "greatest thing since" gold standard has changed from bacon to marrow. Despite this heretical switch, please don't discount me as a soldier in the war against cupcakes. Or sliced bread.
Best thing since sliced bread? The knowledge of how BAD it is for you. Thank God in our age of instantly available technology we now are no longer held to ransom by agro-conglomerates hell bent on their own profits at the expense of human health.
If not that, then probably full English Christmas Dinner with all trimmings ;)