Andrew Bonar Law was the Leader of the Conservative & Unionist Party - and so also Leader of the Opposition - in August 1914. He may have been too Unionist for the more radical Liberals to fully support a Coalition, given in their eyes how he had almost openly called for a rebellion in Ulster over Home Rule.
Balfour pops up as someone not so loathed by the Liberals, but he had almost split the Tories during his Premiership, and had stepped down as Tory leader following a great deal of strife over the House of Lords battle, so I think it unlikely his own party would crown him PM.
Joseph Chamberlain has just shuffled off this mortal coil and Austen is really still known as Joe's son, so not really a contender. Carson is even more virulently disliked by Liberals than Bonar Law. Lee is sometimes seen as a lost leader but IMHO was no more than adequate. Possibly Lansdowne as being a peer probably not quite an issue then? Curzon even assuming he's not too bruised by his battle with Kitchener - possibly more to Liberal tastes?
And with Lloyd George yet to make his name at Munitions, he is probably even more out as a Coalition PM given battles over the People's Budget, Lords' reform and the Land Tax will have coloured Tories views of him (took a World War for them to change their minds).
Depends what your POD is. How violent has the Irish Question turned? Has Lloyd George had the opportunity to push through most of his radical programme? Did Balfour defeat Asquith over the People's Budget or the House of Lords and so did not step down - in which case he would be the obvious choice as PM is he is not already.
And what are the numbers in the House of Commons. Labour & the Irish Home Rule parties held the balance. Have they lost seats to the Tories, or have the Liberals slumped after the King refuses to countenance creating new peers & their morale dives?