I was trying to find if there's an Android phone that was officially updated for more than 2 years and came up with this list, mostly based on data from Wikipedia articles. Some of the phones have multiple versions and not all of them were updated, while some of the updates were delayed by carriers. I only included the date when the last update was released. Recent phones like Galaxy S4, HTC One, Nexus 4 aren't included.
Most phones from the list got 2 major updates and were supported for less than a year and a half, but these are the flagship devices. For now, it looks like Motorola Droid is the only phone with more than 2 years of official software updates. It's followed by HTC Thunderbolt, with 23 months of updates. Nexus S is close behind, but its updates were timely. Nexus S was updated to Android 4.1 in October 2012, while Thunderbolt was updated to Android 4.0 a few months ago because of Verizon's delays.
Note: Dates are in the dd/mm/yyyy format and dd is always 01. It's a workaround for Google Sheets, which doesn't support the mm/yyyy format.
Thanks, Luís Miguel Viterbo, JD Davison and everyone else who contributed to the spreadsheet.