Hi,
I have the Fuji M adapter. It was expensive as adapters go, but it is without any doubt the best fitting adapter I've ever bought. It's absolutely rock solid and I like the quick access to the lens selection and correction menu.
I use more native Fuji lenses than any third party brand but do have quite a few off-brand lenses. In M mount I have the new Voigtlander Super Wide Heliar 15/4.5 III. I've only just bought it, but on first use, it appears to be excellent. I also have the Voigtlander 90/3.5 Apo Lanthar which I bought before the Fuji released their 90mm, and which I've kept because it's both so good and so small. (Mine's in Nikon mount, but the same optics are around in M mount). Others - I've often been tempted to buy either of the two Voigtlander 21mm lenses - the older 21/4 Color Skopar, because it's so incredibly tiny compared to the Fuji 23mm and wider than the Fuji 27mm, and more recently the 21/1.8 Ultron, which was designed specifically for digital.
I haven't bought genuine Leica lenses, partly because of the expense, partly because I doubt they would offer a lot over the high grade lenses that Fuji offers, and partly because I'm told that many of the wider lenses have corner color and corner smearing issues on digital. Their 24/3.8 would be the first I'd look at...... it sits in among the bunch of 21, 23, 24, 25mm MF lenses I mull over trying one day......
Cheers,
Rod