My wife and I are taking a trip to Germany and Belgium later this year, and I'm tempted to take just one lens with the R5m2. But I can't leave the RF 45mm f/1.2 at home because ... well ... because it's f/1.2.
As it happens, I have recently acquired a good selection of Canon FD and Minolta MD prime lenses.
Canon FD: 24mm f/2.8, 50mm f/1.4, 100mm f/2.8, 135mm f/2.8
Minolta MD: 28mm f/2.8, 45mm f/2, 50mm f/1.7, 58mm f/1.4
I'm leaning toward the 28mm, only because I've never used that focal...