As of today I would probably say the Motorola Droid. It was the first Android I saw really marketed heavily and was so popular people would ask, "Is that a Droid?" instead of "Is that an Android?".
However I think in retrospect, a few years from now, we might be saying the Nexus 4. It is the first phone that I have seen even serious Apple fans make the switch for and then commence heaping praise onto it.
Between the two, the Nexus 4/5 & the SGS3/4 could very well be the phones that usher in the second Android revolution.
Note that thoughts expressed here are merely my personal opinions and will hopefully "There are in fact two things, science and opinion; the former begets knowledge, the latter ignorance." ~ Hippocrates
"If anyone's head is acting an antenna, seek help or wear tin foil." ~ EarlyMon
share my thoughts on the matter to stimulate a respectful discussion.