There is something of a consensus on the Leica forums that the best currently available is the B&W MRC filter. Multi-coated, easy to clean. I only own one, a 105mm YG for my 5x7 lenses, which I don't normally shoot into the sun, so I can't give you a real life comparison. All my filters for 35mm are older and I frequently have to shoot without a filter, because I know I'll get flare, otherwise.