SELECT p.rut from ProvEventServs pes join Proveedors p on pes.prov=p.rut group by p.rut having count(*)>=all(select count(*)
from ProvEventServs pes join Proveedors p on pes.prov=p.rut group by p.rut)
var ret = from pro in db.P
join pes in db.servicioProveedorEvento on pro.rut equals pes.prov
group by pro.rut
db = clase de contexto;
P = Proveedor
servicioProveedorEvento = ProvEventServs