Halo 3 is my game of this geneneration thus far. And only game this gen that goes in to my top 10 games I've ever played. Its one of the few HD console games that made sure group play and high fun factor are always present...and not really put much emphasis on things like draw distance, polygon counts in models and amazing textrures...sacrificing goodies like split screen and coop.
I have not played killzone 2 much but from what Ive seen and played, it will never have the charisma, universal appeal and sheer fun factor Halo 3 does.
Don't get me wrong, Killzone 2 is one bad ass shooter...but it is more comparable to COD4 than halo 3. There is nothing else like Halo 3 available this gen IMO...other than maybe L4D which is similarly unique in its own way.
Now as far as sales, I dont think higher selling games always represent better quality games...but in this case, I agree with this logic.