The current mayor, Sadiq Khan, is, unsurprisingly, the favourite to win re-election. Paddy Power is offering odds of 3/10 (76.9%) on him. It is also offering 7/2 on Conservative Shaun Bailey (22.2%) and 50/1 (1.9%) on Sian Berry, the Green Party candidate.
Although Sadiq Khan has come under fire for the rise in knife crime, he is generally popular with Londoners. A December YouGov poll suggests that, on balance, he has a positive approval rating. The same poll suggested he would easily win in the first round, with 55% of the vote, and has a commanding 24 percentage point lead over Shaun Bailey on second preferences. Boris Johnson’s two terms in office show that it is possible for a Conservative to win in London, but the fact that Labour got 55% of the London vote in the 2017 general election shows that the Conservatives face an uphill climb.
Indeed, there seems little in the way of a Sadiq Khan win unless he decides not to run for mayor again. In that case, one of his deputies, such as Heidi Alexander (20/1), would be the favourite. But if Khan wanted to move back to Westminster politics, it would be very difficult for him to find a seat, so I think he will run. I’m going to suggest that you just take Paddy Power’s offer on him being re-elected mayor at 3/10.