Have a think about this. It is a (relative) piece of cake to transfer bags on a point-to-point flight. Take Ryanair, for instance. It is flying from (say) Hahn airport, where there are a handful of flights a day, and FR are the biggest operator. So any bags will probably be going to FR, there isn't a great deal of space in which they can get lost, so the bags are almost certain to end up on the flight to STN. Once they are at STN, it is a bigger airport, but all bags are either leaving or arriving, and if it is tagged for STN, then it will end up at the arrivals hall. Same goes for charter flights. The other European carriers don't have the level of interlining that BA and bmi do - though I would guess that KLM, AF and Lufthansa, all else being equal, would tend to come out not so well.
For Easyjet, operating between large airports, but still only point to point, again at least all bags will be leaving or arriving, so they are either going to an aircraft or to arrivals.
Where the rubber really hits the road is when you have connecting flights - with different carriers, on different sorts of routes, between large hubs. The potential for a bag to be missing and difficult to track down is huge.
bmi "lost" my bags when I flew from IAD to LHR via MAN, and they both turned up at our house in taxis within 24 hours of us getting home. A nuisance, but not much more. But think about the journey they had to make .... from checkin at IAD to the aircraft; from the aircraft at MAN to a transfer hall and thence to the domestic aircraft; and then to the belt at LHR. It is hard enough for most passengers to find their way - and they have a brain attached (kind of!).
I've not seen the statistics, but based on this analysis, my hunch is that FR would be best on this table, EZ would be next and BA/bmi worst. And I bet a high proportion of their "lost" bags are through connections. If the BA/bmi rate is around 15 per 1000, then I would expect EZ to be around 8 per 1000, and FR to be around 5. Is this anywhere near right?
An interesting fact: the notional standard weight for a bag on an intercontinental flight is 15kg. On a domestic flight, it's 11 kg. So what is supposed to happen to that 4kg when a bag is transferred from the intercontinental flight to the domestic flight?
ITWSBT!