The election will involve a limited form of preferential voting, in which voters get two preferences; in essence, one's ideal candidate, and whom one regards as the lesser evil. (You can vote otherwise, of course; you could put down your two favourite fringe parties, in order of preference, but then once they had been eliminated, your vote would not have any input into the race between the two major-party candidates who are all but guaranteed to square off for the title.)
The two major candidates are, of course, Ken Livingstone in the Labour corner and, in the Tory corner, Boris Johnson, the outrageously outspoken party clown of the Conservative Party. Johnson is best known for his gaffes (such as talking about black areas as being full of "piccaninnies with watermelon smiles" and asking New Guineans whether they have stopped eating each other yet), though has been uncharacteristically restrained in recent months—some say by his campaign manager, none other than Lynton Crosbie, the Australian Karl Rove who kept John Howard's conservative government in power for 11 years and has since become a sort of soldier of fortune to rightwingers seeking office across the English-speaking world. Or, perhaps, by medication; for the most part, his policy pronouncements have been rather vague and not demonstrated much of a grasp on how exactly he intends to manage London (except for hinting that there'll be less management going on, and more power devolved).
Whom would I vote for? Well, naturally, Boris Johnson—if the election was for the post of Mascot of London, that is. His loveable-buffoon act is considerably more entertaining than anything Ken Livingstone has been able to come up with, and since no more suitable candidates (such as, say, a professional wrestler or Leoncie) are running, he'd be the best person for that job. However, since the election is not about choosing whom you'd most like to have a drink with at the pub but whom you'd want managing the metropolis of London, electing Johnson would be at best dismal, and quite probably disastrous.
Johnson has not revealed much of what he would actually do in office. He has issued a few policy ideas, all of which were very sketchy. There was his plan to replace the (much-maligned) bendy buses with something called "21st-century Routemasters", which nobody has actually seen, though we are supposed to believe that Boris will somehow conjure them up once he takes office; the proposal could scarcely sound less reassuring if we were promised flying Routemasters fuelled by pixie dust. (If—if—one were to take Johnson's word at face value, this would presumably involve allocating a big chunk of London council tax revenue to researching and developing a new retro-styled bus; which would be a jolly good boy's-own adventure, though hardly the most efficient use for tens of millions of taxpayers' pounds.) Other policies are either hopelessly ill-costed (such as the commitment to putting attendants on buses, whose costing turned out to be off by an order of magnitude) or else amount to little more than vague motherhood statements, promises to do something about crime or give power back to the people. If you squint hard enough, and hate the thought of Ken Livingstone getting back in enough, you can almost convince yourself that there is something there to vote for. Though you'd really need to believe that anything would be better than Livingstone.
Chances are what would happen if Johnson got in is that he'd pose for a few photo opportunities, pass a few populist pieces of legislation, play a bit with the giant model train set he has won and then, like Toad of Toad Hall, get bored of it and go off to pursue the next distraction. He already would be dividing his time with a seat in Parliament if he got in, so London couldn't expect more than half of his time. Of course, he would remain Mayor of London, but the real decisions would be made by the men behind the curtain, i.e., various nameless apparatchiks.
What these would be we can only guess at. While Johnson hasn't spoken about abolishing the successful Congestion Charge (a policy left publicly to the extreme right—the isolationist UKIP and the neo-fascist BNP—one of whose candidates is also on record as saying that rape cannot be an ordeal because women enjoy sex), he has hinted at rationalising it. It's quite likely that the rationalisation could involve filling it with the sorts of loopholes one could literally drive a SUV through. The recurring theme of "power to the people" could indicate dismantling a lot of the powers of the Mayor of London's office, the breaking up and privatisation of Transport for London, and could mean the end for projects such as Crossrail or the ongoing renewals of the Tube. As for cultural diversity, while Johnson (whom Crosbie has undoubtedly coached well in the art of dog-whistle politics) hasn't outspokenly condemned multiculturalism or championed a Daily Mail-approved brand of John Bull Englishness, he has in interviews said that the best thing about London's diversity was that one could find mangetout in the supermarket. That may or may not mean anything, but it doesn't sound promising. And let's remember that Lynton Crosbie's previous client also seemed like an affable moderate until he was safely in office and the gloves came off in a vicious, dirty culture war.
So yes, I'll be putting Ken Livingstone second. I'm still not sure whom I'll be putting first (I'm leaning towards either Siân Berry or Brian Paddick), though it won't be BoJo the Clown.