I don't fully understand what the difference is between these different ratios.

Could someone tell me what its all about and what the best one is?

 

There is no ideal ratio for all occasions. You want to match the ratio to how you drive most often and what type of equipment you have. Lower ratios (higher numerically) are good for pulling. Higher ratios allow your engine to work less and usually give better fuel economy. You want to keep the engine in the "sweet spot" where the engine is putting out good power and torque. That's usually somewhere between 2500 and 3500 RPM. With a stock set up, 3.73 rear ratio and 27.5" tires your engine will be turning something like 2600 - 2800 RPM at 70 mph. If you change tires to let's say 30" the engine rpm at 70 will drop to about 2100 to 2200 rpm. That 500 rpm drop will cause your engine to work harder because it's not putting out as much power but still has to overcome things like wind and rolling resistance. So to get the engine speed back to 2600 rpm you might have to change to 4.11 or even 4.56 gears.