Hey Koushik,

Congratulation to you first of all for your amazing result. Regarding your questions I'm sorry but I don't think DU provides non college for B.Sc programmes. It's only for BA and B.Com. 

If you wish to apply for B.Sc Mathematics you can go for the normal admissions. I'm not sure if you can make it to the best colleges but you might make it to the average colleges by the end of 4-5th cut off. 

Also please note, if you're taking B.Sc Mathematics, they won't consider best of four. They will consider best of 3 main subjects, that is Physics, Chemistry and Mathematics. But if you wish you apply for other branches other than B.Sc then they'll take best of 4 excluding subjects like Physical Education, Music, Fine Arts etc. 

I wish you all the best!