Very good advice. I'm usng a Panasonic 20 mm as my go to lens and the Leica 15 mm on micro four thirds. That's exactly a 40 mm and 30 mm equivalent. With the Mamiya 7 I also found myself using the 80 mm extensively, which gives you about the same field of view on 6x7 as a 40 mm lens on 35 mm film of full frame equivalent digital.

50 mm is good for portraits. If you're doing a lot of portraiture, use a 50 mm as your go to normal lens.