Hi
1) Since the property was originally a lease hold which was later converted in to free-hold , the best option would be that your grand father, parents, your younger brother and your aunt execute a registered settlement deed in your favour .
2) In general, a family settlement deed need not be registered where as partition deed has to be compulsorily registered.
3) But in your case, since all the members are foregoing their claims, it is mandatory that you should have a registered settlement deed as stipulated in Section 17 of Registration act and Section 53A of transfer of property act.
4) Though gift deed or WILL could have been the other options, both gift deed and WILL have their own challenges in case there are any future claims.
5) In the event of a Gift deed/WILL , the legal heirs (i.e your father, your aunt, your brother, your cousins) or any other 3rd person can always submit a mischievous claim that that lease hold property was allotted to your grand father and his family members and hence your grand father alone was not the absolute owner and hence he did not have the right to execute a WILL/GIFT of the entire property to you.
6) A settlement deed executed during the life time of your grand father by all the members of the family namely your father, your aunt, your brother in your favour is the best legal option available to you.
7) Stamp duty is at 2% of circle rate
Hope this information is useful.