The M7 if you want aperture priority auto exposure. MP if you want a more traditional manual Leica. Both of them are expensive, being the newest models. The M5 is the least traditional of all Leicas. It's bigger and doesn't follow the traditional Leica body design. Its meter system is kind of odd compared to later models. I like the M6. It's a traditional Leica with a meter. It's more readily available on the used market and the price is considerably lower than the M7 and MP.
Beginning with the M4-2, Leica started using more economical methods of manufacturing. The M3, M2 and M4 are felt by Leica enthusiasts to have more hand fitting and to be better cameras. Personally, I just like using Leicas because they're small and quiet. I've owned an M4-P and I currently have two M6's. Except for the meter being dead on one of the M6's, I've had no complaints with any of them.