Ill Do The Answering for this too.
Swampert: Powerful Tank, Though Slow, Its Just about a Perfect pokemon for a lead, it can have great coverage. Plus its good for Tanks to have few weaknesses, making Swampert Perfect. With Its Nice Moveset and Great Movepool in Both Special and Physical Feilds, with quite a few weaknesses, including the popular Electric Types Like Zapdos. Base 90 in Both Defenses is incredibly useful, as it can resist quite well. It Can Have AMAZING HP, and Attack. A Great Tank.
Milotic: A Special Tank. Good SA, and SD, and HP, Not a very good movepool though, making it more Defensive than Offensive. Not a very good Physical Tank, and I'd say Swampert is Better due to Higher Defense AND Special Defense total. But Milotic is an Overall Better Special Tank.
Empoleon: Damn, This Things almost as Underated as Torterra.... Its Not as good as torterra though at all, but Still, Nice Resistances and a good Special Tank, with Not Bad Defense Either, and HP is ok, too. Slow, Just as Swampert, with an ok movepool. Too Bad most of its moves are physical, and its Special Attack is WAY Better. But it is A Steel Type, and Quite Good actually.
Overall: Swampert is the Best Tank and Wall for overall Defense and Special Defense with its High HP. Milotics a Better Special Tank, and Empoleons an all right tank with all of its resistances. Overall, I Would Go With Swampert for its decent SD AND Def with Great HP.