...Undoubtedly Jim Murphy, leader of the Scottish Labour Party (above). And, if things fall a certain way in May, perhaps Nigel Dodds, the Democratic Unionist Party leader in Westminster.
Why? Because we're shaping up for a really, realy tight election. It's hard to see where we are right now nationally: ICM, usually the most accurate of our telephone pollsters, give Labour a 5% lead, though Ipsos-Mori give the Conservatives a three-point advantage. The latest round of Conservative peer Lord Ashcroft's polling is out today, and we've got to the 'bite point' where the Conservatives seem to be able to defend English marginal seats against their Labour challengers. Well, we've got there at Conservative majorities of between 7% and 8% - in Warwick and Leamington, for instance, which it looks like the Conservatives will be able to hang on to, though not so much in London seats such as Ealing Central and Acton. That would probably put Ed Miliband, Labour's UK leader, in Downing Street as the leader of the single largest party, though only just. Ealing is 56th on the party's target list, so Labour might be around the 300 seat mark if they do take it, but miss out on a few supposedly 'easier' gains. All things being equal.
Except that they're not equal. Because Labour seem to be doing rather better in English marginal seats than they are in what one might once have called their 'heartlands' - Wales, where Labour's performance this year has been underwhelming to say the least, and Scotland. That's possibly and in part because the United Kingdom Independence Party (UKIP) are stealing ex-Conservative votes across middle England more than they are Labour's (though they seem to be biting chunks out of Labour's support in the north of England as well).
Labour faces by far its most formidable challenge in Scotland, from a rejuvenated Scottish National Party that is building on the back of its grass-roots recruiting and activism during the Scottish independence referendum. Right now, its membership nudging 100,000 (far more than the UK-wide Lib Dems, for instance), the SNP senses a historic opportunity to win the most seats in Scotland (i.e., more than thirty) and to declare themselves representatives of Scotland as a whole at Westminster. The data is again not completely clear, but right now the massive SNP surge in the polls would suggest an unprecedented number of gains for them: of 40 or even more Parliamentary seats. That still seems unlikely in practice, partly because a lot of the Labour majorities they face are so large, but they've done it before in elections to the Edinburgh Parliament, so they can (at least in theory) do it again.
That's where Mr Murphy comes in. He's tough, experienced, unafraid of a fight - and very, very ambitious. He's a plausible and outspoken leader who's in a good position to unite pro-union forces around a centrist Labour Party that still talks about social justice. He's a bit right-wing for many Labour activists in Scotland, and for many of the left-leaning commentariat there, though that never hurt Tony Blair in England (or in Scotland, come to that). And he's going to have to be on his game, and more. The data tells us only that the race to be largest party is still a toss-up. Every single seat might count. If Labour loses 20 to 30 of its Scottish MPs, it has absolutely no opportunity whatsoever - a close-to-zero chance - of getting over the threshold to Downing Street. The Conservatives will be the largest party, and they'll go occupying the great offices of state, albeit in a badly wounded, last-man-standing kind of way.
That's where Mr Dodds comes in. For the Conservatives have been doing quite a bit of under-the-radar wooing of his Democratic Unionists over the past year or so. They know that even their own numbers, plus those of a severely-depleted Liberal Democrat grouping, might well not now add up to the effective majority post of 323 - let alone the 326 needed for an overall majority were Northern Ireland's Sinn Fein MPs to take up their seats, or the 330 or so one might want to be confident about actually governing for any length of time. They'll need the eight or so DUP MPs then, won't they?
Forget about David Cameron and Ed Miliband. The most important party leaders of 2015 are those who the people of England probably never give a thought to, even if they've heard of them.
