Regret selling my first M, a Leica M6. I had just snagged a great deal on an M4/ 50mm Elmar combo I couldn't refuse, but with an M6, M4 and CL, something had to give, as I both had too many M-mount bodies and also sought out an M3 for the higher magnification finder with the majority of my shooting being with a 50.
I shoot with an M3 and M4 now, but prefer it was an M3 and M6, as I miss the black body, 28mm frameline, and possibly the light meter. I don't miss the rf patch flare of the M6 though.
One day, I will swap my M4 for an M6/ M4-P one day, and have the 50, 90 & 135 framelines scrubbed. That would be the perfect compliment to my M3, for me.