These 3 cameras are of 2 different categories. Depends on what you want to shoot.
C-750 and Z2 are ultrazoom compacts. Thier 10x zoom lens gives you very long reach. If you're interested to shoot distance subjects then these models will suite you better. I'm not sure abt Z2 but C-750 produces very good picture with very low CA (which is common problem with ultrazooms), Solid metal body.
G5 is a prosumer level digicam with 4x Zoom lens. Canon G series well known for excellent picture quality, good low light focusing and features fast lens.
G5 should be atleast $300 more than the above models. This should be a good choice for normal travel, portait, event picture taking.