So attack priority is based on two factors: ATP and OTP. ATP is used by default, with OTP used as a back up if other units are close. Higher # = Higher priority. During beta units with an OTP of 0 were calculated the same as other OTP numbers. This changed in one of the patches to have the OTP of 0 mean use atp value.
This list can also be found on Liquidpedia
Unit ATP OTP
##Protoss## Archon 20 0 Carrier 20 0 Colossus 20 0 Dark Templar 20 0 High Templar 20 0 Immortal 20 0 Intercepter 20 0 Mothership 20 0 Observer 20 0 Phoenix 20 0 Probe 20 10 Sentry 20 0 Stalker 20 0 Void Ray 20 0 Warp Prism 20 0 (identical for both modes) Zealot 20 0 Force Field 0 Forge 11 Fleet Beacon 11 Gateway 11 Cybernetics 11 Dark Shrine 11 Nexus 11 Photon Cannon 20 Pylon 11 Robotics Bay 11 Robotics Fac 11 Stargate 11 Archives 11 Twilight 11 Warpgate 11
##Terran Auto-Turret 20 0 Banshee 20 0 Battlecruiser 20 0 Ghost 20 0 Hellion 20 0 Marauder 20 0 Marine 20 0 Medivac 20 0 MULE 20 10 Point Defense 20 0 Raven 20 0 Reaper 20 0 SCV 20 10 Siege Tank 20 0 (both modes) Thor 20 0 Viking 20 0 (both modes) Armory 11 Barracks 11 Bunker 19 Command Center 11 Engineering 11 Factory 11 Fusion Core 11 GhostAcademy 11 MissileTurret 19 OC 11 Planetary 20 Reactor 11 Starport 11 Supply Depot 11 Tech Lab 11
Note: Flying does not affect priority
##Zerg Baneling 20 0 Brood Lord 20 0 BL Cacoon 10 0 Broodling 20 0 Changeling 20 0 Corruptor 20 0 Creep Tumor 11 0 Creep Tumor 19 0 (Built) Drone 20 10 Hydralisk 20 0 Infested 20 0 Infestor 20 0 Larvae 10 0 Mutalisk 20 0 Nydus Worm 11 0 Overlord 20 0 Overseer 20 0 Overseer 10 0 (Cocoon) Queen 20 0 Roach 20 0 Ultralisk 20 0 Zergling 20 0 Baneling Nest 11 Evolution 11 Extractor 11 Greater Spire 11 Hatchery 11 Hive 11 Hydralisk Den 11 Infestation Pit 11 Lair 11 Nydus Network 11 Roach Warren 11 Spawning Pool 11 Spine Crawler 20 Spine Crawler 19 (Uprooted) Spire 11 Spore Crawler 19 Spore Crawler 19 (Uprooted) Ultralisk Cavern11
note: Burrowing does not affect priority