Deductions can be made in this order:  In row 6 only b6 can be F, and so in row 5 only a5 can be F; in its region c2=F; d1=F; e3=F.  In it region f5=A, e4=A, d3=A, b2=A, c1=A.  So e2 can only be B, f2=C, in row 1 a1=C, in row 2 a2=E, and a3 can only be B.  Then a4=D, d4 can only be E, b3=E, f3=D, b1=D, b4=C, c4=B, f1=B, f6=E, c5=E, c6=D, d6=B, d5=C, e6=C, e5=D.