select imię, nazwisko, p1.KrajOdbiorcy from
(select count(IDzamówienia) as ile, IDPracownika, KrajOdbiorcy
from Zamówienia as z
group by IDpracownika, KrajOdbiorcy) as p1
join
(
	select max(ile) as ile, KrajOdbiorcy from
	(
		select count(IDzamówienia) as ile, IDPracownika, KrajOdbiorcy
		from Zamówienia as z
		group by IDpracownika, KrajOdbiorcy
	) as t1
	group by KrajOdbiorcy
) as t2
on p1.KrajOdbiorcy  = t2.KrajOdbiorcy
join pracownicy p on p.IDpracownika = p1.IDpracownika
where p1.ile = t2.ile