The 2di I am closest to clinching would have to be Interstate 16, of which I am only missing the part east of I-95.
After that, second place probably goes to I-30, as I believe I have been on all of it except the section west of Fort Worth, and part of it in Little Rock.
Third place is probably Interstate 24, as I have been on all of it except the remainder of it northwest of Cadiz, Kentucky.
In the case of clinching an interstate within a state, I am very close to having clinched all of I-75 in Georgia, but I am missing the section in Macon in between I-16 and the southern interchange with I-475.