Don't think London-Paris is that big a market any more - Eurostar hold something like 65% of the market share on that route. I believe London - Amsterdam is probably slightly larger - around the same number of scheduled flights plus the LCCs as well.
Statistics published by the EU show that Dublin-London is the busiest air route in Europe, with 4.4 people travelling between the two cities each year. It also emerged that Dublin is the eighth busiest airport in the Community, with 11.6m passengers in 2000.
Madrid-Barcelona was 4.16m pax in 2001. So, yes it would win if you were talking about airport pairs rather than city pairs. But as the topic says "route" I take that to mean city pairs - others' opinions may differ.