That's really completely your own decision. Everyone is different.
Personally, my favorites and most useful and usable mf build characters include,
MF Cold Sorc (Blizz/Orb)
MF Cold/Lightning Sorc
MF Hammerdin
MF Light Sorc
MF WW Barbarian
You can make anything an MFer thought. I've had some real interested Amazon MFers and Druid MFers