I am currently studying in a private law school in Delhi. Till which NLU will a drop year be worth it? (I have chances for TOP-5 and will definitely get top-10)

Only till GNLU NLIU level I think. Depends on how bad your private law school is though. Symbi JGLS etc. I wouldn't recommend but if its something like Amity that had degree recognition issues then do change for the top 10 NLUs.

For top 5, do it. Top 10 not worth it.