I ride them all here...

For getting around my "as flat as you want it to be city" I can ride my old Raleigh club bike with it's SA 3 speed and maintain good road speed and still manage some decent grades.

IGH hubs need very little to keep them running smoothly as they are sealed against the elements and as such remain highly efficient in the worst environments that will cause degradation in derailleur equipped bikes which do require more cleaning and service and do not enjoy the same working life. The old SA AW is said to be good for 50,000 miles which is something no derailleur equipped bicycle can match.

My fixed gear and ss bikes are also geared to give that nice balance of speed and hill climbing ability and have a little edge over the three speed as even my fixed gear mtb is lighter.

For simplicity I have a Peugeot UO8 with a simple 6 speed derailleur setup and it will take me anywhere but does lack a little top end and will be getting fitted out with a double to make it a 12 speed.

When I want to go faster and know I will be dealing with longer hills and longer distances where I might be battling strong winds I ride my hybrid which is a 24 speed or my 24 speed touring bike.

As far as derailleur gears go... I am thinking that no mortal needs more than 8 speeds in the rear and for most a well set up 6 speed would be just fine.