The poetry of Sri Sarab Loh Granth mainly consist of Bir Raas and Rudra Raas style of poetry. Meaning the poetic style is very passionate and militant. Sri Sarab Loh granth is divided in Raags and Chaands, but many of the Raags of Sarab Loh grant no longer exist today. For example , some raags were borrowed from musicians from the Mughal courthouse but these old raags no longer exist. In regards to Maalkauns Raag I dont know much about its history, but I dont think it was Sri Guru Jis favorite raag as there are few banis of this raag in Sarab Loh granth. If I remember correctly I think Sorath Raag is the most popular raag in this granth. Either way I dont think Sri Guru Ji had a favorite raag; bani was based on the message not the raag. The raag was there just for the tempo of the bani. When Gursikhs listen to Gurbani they dont listen to the raag they listen to the Bani. The raag is just the background music.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥