using (NomeDataBaseEntities context = new NomeDataBaseEntities()) {
var risultato = (
from mt in context.Mi_Tu
join m in context.Mi on mt.IdM equals m.IdM
join g in context.Gr on m.IdGr equals g.IdGr
join t in context.Turni on mt.IdTurno equals t.IDTurno
where t.IdTipo == 1
group new { g, m, t } by new { g.gr, m.co, m.no } into grp
orderby grp.Max(u => u.t.Data)
select new { grp.Key.Gr, grp.Key.Co, grp.Key.No, Ultimo = grp.Max(u => u.t.Data)}
);
}