A monthly metrics report from mobile advertising firm AdMob has revealed that, in its listing of top ten Android-based smartphones by market share, the Motorola Droid notched the top spot as the most popular Android handset; followed by the HTC Hero. The Google-branded ‘Nexus One’ handset, incidentally, was at the tenth spot on the list.

However, since the AdMob statistics were based on the 12.7 million Android handsets in the AdMob network in May, the June-first-week-released HTC EVO did not figure on the list.

Going by the data released by AdMob, while nearly 21 percent of the Android handset owners have the Motorola Droid; 16 percent of the Android users have the HTC Hero; and barely 2 percent of them have the Google Nexus One.

The AdMob data shows that the November 2009-launched Motorola Droid – which was made available exclusively on Verizon Wireless – has managed to retain its strong position in the market, despite the noticeable flurry of Android handsets being released each month.

Apparently, the only Android-based handset that has the potential to pose some serious challenge to the Droid is the last-month-launched HTC EVO, which is available exclusively on Sprint. The market-share scuffle between the Droid and the EVO would be an interesting one – more so because Sprint has a comparatively smaller marketing budget and fewer subscribers than its bigger rival Verizon, which carries the Droid.