Well as an avid reader of Horus Heresy novels, it is pretty hard to make up my mind, it all depends on my mood as each one has its benefits.
For least favorite, i’ll definately go with one of the anthologies such as "Age of Darkness", but if that doesn’t count I’m pretty much out of the picture. In my eyes "Pharos" is quite weak but not necessarily the worst as you understand it, it simply deals with a less interesting aspect of the Heresy, but only because I’m not really into the Ultramarines.
As for best, depends on what you enjoy, for pure action I’d go with "Betrayer", for more subtle plots I recommend the Dark Angels-cycle, especially "Fallen Angels" and " Angels of Caliban" as they have multiple plot lines which are all very interesting.