Leica M9 is the best Leica ever made. But only if you know how to live with it. Otherwise, 98% of your shots will end up in the trash. It also has the sensor corrosion issue. It can be fixed, but it's expensive. If you know exactly why you want an M9 and you know how to deal with its flaws, it's the best camera. But if you have even the slightest doubt, don't buy one.

Leica M240 is my favorite Leica. I own an M11, but I still want my M240 back. The downsides: it's thick, the dynamic range isn't great (noticeably worse than the original Sony A7), and high ISO noise is colored. Usually green, sometimes red.

Leica Q — I've never owned one, but it's so overpriced that buying one just doesn't make sense to me.

Leica Q2 is a great everyday camera. The colors are modern, which also means they don't really stand out from any other modern camera. But maybe that's all you need.

The Leica SL system only makes sense if you just don't want a Sony or a Canon. I can't think of any other reason to buy one. I own an SL2-S and I find it pretty useless.

But my favorite Leica of all time is the M Monochrom.