Kal ho naa ho is one of my most fav movies of srk, but I'll choose hrithik for kmg, it was a very tough role and it came at a time when hrithik was struggling very badly. Though honestly speaking it doesn't make sense to compare a romantic movie with a Sci Fi film.
Don2 and dhoom2 again can't be compared. Hrithik was cool and stylised but srk had darker shades to his character in don2. If i had to choose i would say srk in don2.