Okay, I'll bite. Because that question can have as many answers and opinions as there are people here, but: I chose an M6 Classic because -
1. It has a good meter. Some may disagree, but I think it's fine. Actually, I think it's excellent for the kind of work I do. (I'm not a 35mm zoner)
2. It is still a completely mechanical camera, ultimately an M4 with a meter. M4 would have been my first choice, but after having done some research and considering the similar cost of an M4, I like the bonus of not having to estimate exposure or tolerate an awkward little hotshoe meter. I like that if batteries go out, I don't have to stop shooting.
3. MP is very expensive and I think I'd hesitate to use it and treat it the way I do my M6.
4. The M6 I have has the MP VF mod ($250 I'm told) and while I can't directly compare to one without, I think it's probably a significant improvement and one of the reasons to choose over the MP itself.
5. M3 would be nice, but I like black and black paint would be prohibitively expensive, more a collector piece than a user and again, no meter. If in-camera metering REALLY isn't important to you, than M3 or even M2 might be a nice choice.
6. Not specific to M6, or maybe even Leica, but I love shooting film with a completely manual camera. I think it was Mike Johnston (and I'm sure many others) has said: It has everything I need and nothing I don't to make pictures.
What it does have is so refined and so simply and logically put together that once one has gotten familiar with it, it really is a camera for life. It works that way in my hands and it is built that way. I could go on for pages, but in this world of disposable digital technology I love having something that feels connected to the past (camera is from 1984, but could easily be from 1948) and literally feels like it's machined from solid piece of steel. Dials and all. I rarely "fondle" it anymore the way I did during the 'honeymoon'. It is after all just a tool, but it is a supreme tool (to parrot Leica's marketing jive; that one seems to fit). I use it now with confidence and aplomb and I think it shows in my photos. Especially the discreet street stuff, where the Leica REALLY shines.
If you are one of those that happen to be into it for the Leica status, don't do it. I only say that because enough time on the forums talking and reading about Leica and you start to think that 'cult' status is the motivation for many, if not most. In reality, VERY few people recognize it for what it is and mostly think you are some kind of an anachronism for toting around an outdated looking film camera while they sneer at you with their D3s and 5DM2s with foot long lenses. I keep to my own, private pleasure at times like that.
I won't ramble any longer now, as I'm sure I've thoroughly given my opinion with regard to your original question about a paragraph ago. All I can say is, if you enjoy the type of photography the Leica RF is best suited for, you probably won't regret any particular M model that you choose. I haven't has GAS in years now and that alone is worth something in my book. I just take pictures now.
Best of luck to you!