I've got a few long term non-con rivalry games, like Kentucky St, Shaw, USC Aiken and S Connecticut St (all of which I will have played 20 or more times after the current Smith season) with Henderson State and Puerto Rico, Cayey and Tusculum with Bryant, but the best DII rivalry I've been a part of, based on coaching consistency, overall success and quality of play and sheer volume of meetings, is between division mates Bryant and St. Rose in the North Division of Naismith's Northeast-10.
Minus a 3 season break between seasons 22-24 Chovy24 has been at St. Rose since season 11, I've been at Bryant since season 19. St. Rose is #1 in Naismith DII in both wins and NTs, Bryant is #3 in both wins and NTs. They are the only 2 schools that have won more than 100 NT games, nobody else has won 80.