By Mark Lamport-Stokes PACIFIC PALISADES, California, Feb 17 (Reuters) - World number one Jordan Spieth and third-ranked Rory McIlroy head a top-class field for this week's Northern Trust Open at a venue widely regarded by players as one of the best on the PGA Tour. American Spieth has long rated the par-71 layout at iconic Riviera Country Club as one of his favourites while Northern Ireland's McIlroy is playing the event for the first time this year, largely because of what he has been told about the course. More...