Board Thread:Warhammer 40k General Discussion/@comment-66.206.224.153-20131222032846/@comment-25177957-20140903065439

I'd say the most combat effective chapter would be the Legion of the Damned :P They are an undead, psychic remainder of the destroyed fire-hawks chapter, and they basically cannot be defeated by any enemy of the Imperium with the exception of powerful daemons and sorcerors. However, they only appear at the most dire of dire times.

Now, if we want to narrow it down to specifically codex-compliant chapters I would say ether the Dark Angels, Space Wolves, or the Ultramarines, however the Ultramarines were mostly buffed due to Matt Ward fanboy-heresy. The Space Wolves are quite dangerous even in small numbers (for example they made a fool out of the Ordo Malleus after the first war of Armageddon.) And the Dark Angels not only have effective tactics (ambushing Dark Eldar is quite the accomplishment), but also have a primarch which can be confirmed still alive (although asleep) It's also possible that the lion is psychically guiding his legion/chapter to a minor degree even while sleeping.