Coming from FPS gaming before LoL I must stress that having as little mouse acceleration as possible should be your first priority before looking into sensitivity. Mouse acceleration will modify the distance of your mouse movements depending on the rate at which you move it. Having a constant between the distance you move your mouse and the distance your cursor moves is essential for developing muscle memory which will make your mouse movements more accurate.
-Make sure you have disabled enhanced pointer precision in Windows control panel
-Disable any acceleration options in your mouse's config if it has any.
-You can apply this regedit fix http://donewmouseaccel.blogspot.co.uk/2010/03/markc-windows-7-mouse-acceleration-fix.html which will remove acceleration in windows and ensure 1:1 movement.
If you have minimal mouse acceleration then you will begin to develop muscle memory in terms of your mouse movements as long as you keep consistency. This means often changing your sensitivity settings, surface, grip will affect this development and your long term accuracy will suffer.
People will carry on with the high sens vs low sens arguement to the end of time however it is fact that a lower sensitivity will always be more consistent in terms of accuracy as with a lower dots per inch, it is more reliable in "selecting" specific pixel spaces as you won't be skipping as many pixels than you would with a higher dpi. On the other hand, a higher sens will obviously give you a higher mouse speed.
Someone else recommended that you want the "highest sensitivity you can still use accurately" however I believe it should be the "lowest sensitivity you can still use reactively and can control." It should also be factored in the mouse grip the user is using. Claw/finger grips generally benefit from higher sensitivities while a palm grip will benefit from a lower sensitivity. That's not to say you can't change your mouse grip and surface, and adapt to it.
The actual value, be it low or high does not matter too much in league of legends. What is most important is how comfortable it is and consistency. Pro's will use varying sensitivities, there is no 'best one.'