I think it will probably be 'Skull', because it's the most obvious choice. Personally, I'd like 'Mama Lay Softly...' or 'Black Cloud' because I think they're the most likely candidates for chart success.
Really though, I hate to be pessimistic but I don't think any of the songs on YOR are 'singles'. With songs like 'Irish Blood, English Heart' and 'First of the Gang To Die', you could imagine non-Moz fans liking them. Even though I love Years of Refusal, I'm just not sure it would ever appeal to the people who are making Leona Lewis and Lady Gaga No.1.
So really, Morrissey should just f*** them all and release 'Birthday' or something.