These articles [2] list phones that get frequent OS updates:

• Google/Motorola Nexus 6 (6.2", no removable battery or SD card, upgradeable to Android 5.1)
  I'm not super fond of the phablet trend, but this still fits in my pocket (mostly). And it will indisputably be the most up-to-date.
• HTC One M9 (5.7", no removable battery, Android 5.0)
• Moto X (2nd gen) (5.5", not available on Sprint, Android 5.1)

In addition, these are other phones I might consider:

• iPhone 6 (4.7", no removable battery or SD card)
• Samsung S6 Active (only available on AT&T, and software updates may take significantly longer)
• Samsung S6 (5.6", no removable battery, Android 5.0)

The latest version of Android is 5.0 "Lollipop". The previous release was 4.4 "KitKat".