Ultimately it all comes down to what you consider a rivalry. I consider a rivalry to have 3 elements. First, there needs to be a history of multiple games played on a fairly consistent basis. Second, there needs to be at least some parity in the W-L column. If one side is not winning at least 1 out of every 4 games, keeping in mind the context of history and not slumps, it's hard for me to consider that a rivalry. Third, there needs to be some degree of a fan passion against the other team and their fans. This is usually bred from geography and fanbases consistently interacting with one another. That's why the top 4 is fairly clear with those factors in mind, although the order is debatable.
1. Pitt
2. VT
3. Syracuse
4. Maryland
The 5th spot is hard as I don't see any team that fits. PSU, Rutgers, and ECU are all out because disparity in the W-L column. Marshall is out due to lack of history and W-L disparity despite the obvious animosity between fanbases. UVA is out due to lack of history at all despite geography and fan animosity. Kentucky is in the same boat as UVA.
Louisville had potential in the BE 2.0 days, but not enough history there to count. If WVU joins the ACC, I could see that happening though. Cincinnati has the same problem as Louisville. All the current Big 12 schools lack history with WVU. So I guess I can't really name a 5th for WVU.