I agree with the above as follows:
Black Templars (disclaimer - I am a fan of the BT, but not yet a player)
Iron Hands - most teched out (see the new Iron Hands named character announced).
I also like the "Codex: Last Wall" idea. This would be an alternative to a dedicated Black Templars codex, however. In this context, each of the founding chapters so far less detailed - Imperial Fists, White Scars, Raven Guard, Salamanders, Iron Hands - would be justified in having their own codex. (OK, OK. The Ultramarines can have one, too.)
I would like to see a "Codex: YourGuys", containing rules for making your own chapters as something other than "published chapter with a different paint job". The main challenge there is balance - unbalanced rules would lead to such a codex being heavily reviled, no matter which way the balance fell.