which screwdriver do you guys use to open it ?

 

What are you talking about? The actual key fob? Get a precision screwdriver set or just go buy a glasses repair kit.

 

Widest one that will fit in the crack will do the least damage.

 

There's also a small Phillips head screw right near the metal key shank. A normal Phillips screwdriver will probably be too big. So I'd second the recommendation of a precision screwdriver set that includes Philips head screwdrivers.