decoration like a thilak (a beautiful symbol on the forehead) —

 the best among the brāhmaṇas who are shining like the thilak (vertical symbol) on the forehead —

like thilakam (decoration on forehead) —

 brāhmaṇas who are like thilakam (chief, like the symbol on the forehead), their —

one which is held on the head —

decoration like a thilak (a beautiful symbol on the forehead) —

like a thilak which is applied on the forehead in the form of ūrdhvapuṇdram (upwards pointing symbol) —