No question: it was the Desire (slightly biased as I had one): that was the first 'hit' Android phone. Nothing before it really sold well - no even the original Nexus that it was based on.
The Desire looked good, was reasonably high quality, had a decent spec (for the time, though it was beaten very, very soon afterward) and the O/S was mature enough to start to really compete with iOS. It was also the first 'droid to win phone of the year in lots of publications.
Compared to what came (very soon) after it, the Desire was a bit pants (500MB of RAM was never going to be enough), but it was the Desire that kick started the market and after it, Android went from strength to strength.
The Desire was the Stalingrad of the war with Apple