At this point in the series? House Tyrell.
The Starks are led by little more than a child, and their old patriarch is dead.
House Arryn is led by a sickly child with a crazy mother.
House Tully really have no power left anymore.
House Greyjoy are trapped on their islands and nobody cares about them.
House Lannister has an extremely uncertain succession and all sorts of unpleasant characters.
House Baratheon was completely ruined by the Battle of the Blackwater.
House Martell do nothing but sit in Sunspear and cry themselves to sleep.
House Targaryen is under the control of a slightly mentally unstable child in Essos.
Contrast with House Tyrell, who have just won an enormous battle, whose homeland is untainted by war, who have plenty of eligible heirs, who are allied to the Iron Throne, who are wealthy and have a huge army, and who have lovely food and wine to boot. No contest.
(Former) Lead Moderator and (Eternal) VGC Detective 
