Felt
The cheapest to fit. Modern torch-on felt is far better than the old kind, though it has the shortest life of the four.
EPDM rubber
A single sheet with few or no joints, which is where flat roofs usually fail. It copes well with movement and standing water.
Fibreglass (GRP)
Laid as a liquid over new boards to form a hard, seamless surface. It suits roofs that get walked on.
Metal sheet
For pitched garage roofs, profiled steel sheets replace old corrugated panels and can be insulated.
Old cement sheets
Many older garages have corrugated cement roofs that may contain asbestos. They should not be drilled, broken or jet-washed, and need removing with care.
What else to check
The boards and joists underneath, the fall of the roof so water drains, and the edge trims and gutter.