There are several ways that this can be done:
A filter in the WHERE clause:
select id, name, population
from yourtable
where population in (select max(population)
from yourtable)
Or a subquery:
select id, name, population
from yourtable t1
inner join
(
select max(population) MaxPop
from yourtable
) t2
on t1.population = t2.maxpop;
Or you can use TOP WITH TIES. If there can be no ties, then you can remove the with ties. This will include any rows that have the same population value:
select top 1 with ties id, name, population
from yourtable
order by population desc
Since you are using SQL Server you can also use ranking functions to get the result:
select id, name, population
from
(
select id, name, population,
row_number() over(order by population desc) rn
from yourtable
) src
where rn = 1
See SQL Fiddle with Demo of all.
As a side note on the ranking function, you might want to use dense_rank() instead of row_number(). Then in the event you have more than one city with the same population you will get both city names. (See Demo)