After you spend about £30 or so the internals of mice are very similar. So it mostly comes down to shape and extra features like lighting, more than two side buttons, on the fly dpi buttons
Shape is by far the most important factor imo, what you like depends on how big your hand is, how you hold the mouse etc.
I'd recommend watching a guy called RocketJumpNinja on youtube, he makes incredibly well thought out reviews and does big roundups too.