I-15 between Butte and Helena, Montana would be the first on my list of favorite slab I have ridden. Very nice scenery and some nice sweepers.
I-24 between Monteagle, TN and Chattanooga, TN is not too bad for slab.
Will second the other comment on I-40 between I-81 and Asheville, NC. Great sweepers combined with elevation changes.
Second the comments on I-70 in CO west of Denver. Scenic.
I-77 between I-81 and Charleston, WV gets an honorable mention.
I-64 between I-77 and I-81 in WV/VA gets an honorable mention.
And of course, I-10 in FL....no, just kiddding!