I will say the Aviva Premiership as that is the only League fully played in Britain. Though realistically I would say the Pro 12 (the one the Ospreys play in) as that includes all the Ireland provinces who are extremely strong.
The English Aviva Premiership and the Celtic Pro-12 League (which includes teams from Scotland, Wales, Ireland and Italy) are on approximately the same level. Not much to choose from between them.
