For me it's the 50, hands down, but I'm not you.

I "see" wide. If I could have Mamiya design me a new lens, it would be a 35 or even 30 non-fisheye, ultrawide. I like the 14 to 24mm range in 35mm cameras most and would like the same angle of view in 6x7. But that's me.

Don't let my or other folk's preferences in lenses effect your decisions. What lenses to use really should be a reflection of what you shoot and how. Do you tend to use short or long lenses? Normals? Super teles? Do you "see" the world photographically as if through a wide angle or a normal or a long lens?

The 127 is arguably a very very short tele, or a longish normal, or a moderate portrait lens. What task is the next most important task you want to accomplish? If you see the world "wide" then maybe the 65 is a good match for you. Why? The 127 is a very moderate lens - not too long - not too short. The 65 is also a moderate angle of view.

If you tend to long glass, then the 180 or the 250 would be a good next step.

Best to look through some actual lenses and see if they make sense for you.