[blueblueblue]

I need to find which chloride is the most reactive, when treated with only methanol. 

Since methanol is a weak base, this will favor SN1 reactions.  And the left chloride is a vinylic halide, which cannot undergo substitution or elimination reactions.  So, it will be between the right chlorides. 

I think that the bottom chloride, with an oxygen, is the most reactive because a lone pair of O forms a resonance and stabilizes the carbocation.   But I am not sure because oxygen can withdraw electrons through inductive effect, causing the carbocation to be more positive?

I think I am correct but am I missing something? 

Thanks in advance for your help.

[cundi]

http://en.wikipedia.org/wiki/Anomeric_effect

[Dan]

I think that the bottom chloride, with an oxygen, is the most reactive because a lone pair of O forms a resonance and stabilizes the carbocation.

The resonance effect outweighs the inductive - this is general (as far as I know, can't think of an exception but there may be some) for O or N. For example, this type of substitution underpins the vast majority of the field of glycosylation (in both the lab and in nature).