STN-CRL
Ryanair operated STN-CRL from April 2001 (the opening of the CRL base) to April 2004, carrying over 30,000 passengers a month on a few occasions.
The route was re-opened in June 2007 and operated at least twice daily until March 2008, carrying up to 16,000 passengers a month. In both 2007 and 2008 the route achieved an average flown load factor of 56%, giving it the 2nd worst load factor from STN in both years.
The London - Brussels air market has suffered hugely in the last 10 years, shrinking from 1.5 million passengers in 1999 to 0.5 million in 2011.
With Ryanair we've learned never to say never, but I would have thought that having tried STN-CRL twice before and in the context of a market that has all but collapsed, it would be surprising if they tried a third time.
I suspect that their experiences on STN-CRL also explain why STN-BVA has never been attempted.