Transform/Imposter will copy all of the opponent's stats except for HP.
Look
Cry
Attack Stat
Defense
Speed
Sp. Def
Sp. Attack
Capture Rate
Experience Given when defeated
Any stat changes made by the foe
Moves
These are the things that Transform/Imposter gain from the opponent.
A strategy in Hackmons is to use Blissey with the Imposter ability, as it has the highest HP in the game.