What version of SQL Server are you using?

I am not sure is you are looking for the last non-zero value or the maximum non-zero value. Anyway, filter the rows to get just rows with [ReadNo] greater than zero and then enumerate them in descendent order by DateCreated.

with R as (

select *, row_number() over(partition by DeviceNo order by DateCreated DESC) as rn

from T

where ReadNo > 0

)

select DeviceNo, ReadNo

from R

where rn = 1;