Here's a quick snapshot of defenders, FWIW.
Fighter: They play out like soldiers (e.g. high defense, high attack bonus) with either a secondary striker or controller lean. With a spear + shield style you can get into all kinds of push/prone fun with polearm momentum. If you have the Con for a hammer, as you go up in level you can layer slow and daze on your attacks. Brawler fighters are more single target lockdown since they can grab and easily get prone off of World Serpent's Grasp.
Battlemind: They are quintessential brutes. The lack of a good MBA pulls them down a bit, but spamming lightning rush helps your stickiness and you can tank and tank and tank away and keep on tanking and then tank some more. Once upon a time it was the battlerager fighter that couldn't be bloodied; now it's the battlemind.
Paladin: They minor in healing. They're pretty meh through heroic, but the feat support in paragon and epic cranks it up. Chaladins in particular become insane save-granting machines, have lots of multi-mark options, can become a font of self-generating THPs for themselves, and if you worship Erathis you hand out damage resistance to adjacent allies. Then you hit epic and weakening challenge makes their mark vicious to ignore. However, if you don't have access to Divine Power, they are very, very weak.
Honorable Mention 1 - Knight and Cavalier: Both of these classes are very simple to run out of the box and do a great job in heroic, but the aura in place of a marking mechanism and the poor scaling on the Cavalier's mark punishment makes it very hard to actually defend in late paragon and epic tier play.
Honorable Mention 2 - Blackguard: Yes, I know he's a striker, but if you start with an 18 Str/18 Cha split after racial you can cherry pick a lot of mass Divine Sanction powers from the core paladin and the sanction-boosting feats to give you a mark with fair to okay punishment. You've already got the defenses, this gives you respectable striker damage and a way to generate THP for yourself on top of it.
So, in summary:
Fighter if you want big damage or more control.
Battlemind if you want to tank.
Paladin if you want to off-lead, have Divine Power, and will play in paragon/epic tier or can access Erathis's feat support.