Medic Armour. It administers med-x when you are injured, incresing your damage resistance. It even TALKS! xD
edit by mo: TB51b has more Damage Reduction, slightly more Rad Protection and comes with a helmet and no penalty for your SPECIAL atributes.
The Medic Armor has a little less Damage Reduction, no helmet (u are free to choose a "normal" power armor helmet) and a penalty of -1 for agility. As mentioned before (and you can read it in the wiki as well): It will admister a non addictive med-x variant if your health drops beyond 20% - as long as you have the basic drug in your inventory. So, as long as the drug is used, and another dose is used by the player, the DR is temporary better. The voice and automated system helps against ugly surprises which might offset the penalty of -1 to agility.