Should I use left or right click?
After hooking someone? Right.
You mean the hook combo?
If you hook someone in you should definitely use left click because Right click does limited damage until after a certain range.
However going for a right click+hook+left click+melee is a good combo as well.
Haven’t seen a roadhog post in a while 
Learn the range of right-click. If you can get decent at sweet-spotting it, then you’re good.
1-shot combo is
Right-Click -> Hook -> Left-Click -> Melee.
Some characters may not need the right-click before the hook (characters like Junkrat have large hitboxes), but it’s still a good thing to do before you hook.
Whatever you are comfortable with.
you can do it like me, hook someone, than reload…why click at all… 
Hook -> hello. Repeat as necessary