It really depends on what you want out of the ownership experience. There is no single answer and its totally dependent on what you want from a car, not what someone else needs.
The M has the special engine and immense performance, its an M after all. But its expensive to maintain and run. If thats what you want, thats the best car for you.
The 3.0si is reliable, cheaper to run, still does 80% of what the M does for less than 50% of the price. If thats what you want, thats the best car for you.
The Alpina, seems to have its own niche, it certainly is a rare beast, and maybe thats important to you in terms of an ownership experience.