SQL: how to limit a join on the first found row?

The key word here is FIRST. You can use analytic function FIRST_VALUE or aggregate construct FIRST.
For FIRST or LAST the performance is never worse and frequently better than the equivalent FIRST_VALUE or LAST_VALUE construct because we don’t have a superfluous window sort and as a consequence a lower execution cost:

select table_A.id, table_A.name, firstFromB.city 
from table_A 
join (
    select table_B.id2, max(table_B.city) keep (dense_rank first order by table_B.city) city
    from table_b
    group by table_B.id2
    ) firstFromB on firstFromB.id2 = table_A.id 
where 1=1 /* some conditions here */
;

Since 12c introduced operator LATERAL, as well as CROSS/OUTER APPLY joins, make it possible to use a correlated subquery on right side of JOIN clause:

select table_A.id, table_A.name, firstFromB.city 
from table_A 
cross apply (
    select max(table_B.city) keep (dense_rank first order by table_B.city) city
    from table_b
    where table_B.id2 = table_A.id 
    ) firstFromB
where 1=1 /* some conditions here */
;