I would have to be the <div> tag, as divs have revolutionised the way that websites are designed. 99% of websites, I would say are taking advantage of divs.
Back in the day, we used tables with images in them.
Nowadays, we can do floats and lay out content better.
I'll have to go with <!DOCTYPE HTML> You know what they say, "first impressions are everything", this is the HTML "first impression" if you will An example of an element that has been cleaned up so much from how it was previously, yet has so much meaning.