It's a very tough call. Basketball did achieve something that we haven't done in 26 years and that amazingly even our 89 national championship team and the Fab Five couldn't do so that has to take the cake as more impressive.
For me personally, it is football. The improvement of the defense was incredible not just because of how atrocious we were the year prior but especially after all the metrics on here last summer that said the best we could hope for was an improvement to a defense that was middle of the pack ranked in the 60's nationally.
I don't believe Hoke would have been nearly as successful recruiting so far without our tremendous improvement this last season so as far as importance of regular seasons go I am picking football but for more impressive it is basketball.