I think the most effective defender I had was, bizarrely enough, the strangest one. Dragonborn (fear) Fighter/Snapping Tetsudo. Far from the best Paragon Path (I took it on a dare, lol, as someone said there was no way it could be good), but I got to the point that the DM just gave up.
We had a large battle with a demon and a bunch of minions in the Shadowfell. I moved up, used my dragonfear to mark every enemy on the field (I even actually hit some with it, to my surprise!), used my minor action to use an Intimidate Utility power to give all enemies -5 to hit anyone other than me, action pointed (which gave me a +4 AC boost from Snapping Tetsudo) then full defensed (for another +4 AC).
The DM stopped the game to rant at me for a few minutes, since giving all his enemies -7 to hit anyone other than me, while I had just upped my AC by 8 gave him no good tactical options.
To which I replied, in my best Jack Sparrow impression, "Fighter!". I mean, forcing a Catch-22 was the whole point of the Defender role! And it wasn't like I could do that every turn.
But one turn was all my allies needed to seriously thin out the ranks.