What is the best tool for hanging pictures...correctly?

https://www.youtube.com/watch?v=aBu5o6VY8I8

These videos will help.

https://www.google.com/search?q=hanging+pictures+correctly&rlz=1C1BLWB_enUS700US700&source=lnms&tbm=vid&sa=X&ved=0ahUKEwiygJPb04fZAhUN_mMKHUsOBuQQ_AUICygC&biw=1422&bih=629&dpr=1.13

What do you mean by tool? You need tools depending on how it is hung, by wire, saw tooth hanger, etc. To get the best centering to hang a picture where you want it, you can use a tape measure or one of the laser levels you can buy. Otherwise, a hammer and screwdriver and a marker or pencil for marking placement is all you should need and of course patience and a few choice words for frustration.

A level

Hi CIndy. I use a level and a stud finder for heavy pictures. This allows you to put the nail into the stud instead of just the drywall.

Put very simply a hammer and a picture hook. We have literally hundreds of pictures in our house or anyway wall hung things. We have heavy things and light things. I just eyeball things. Below are two separate corners of the same room. We have plaster walls and irregularly placed studs. If something I hang is "off" I figure out what is wrong and change picture hook to make it right.

I guess I wasn't completely clear, I am a bit OCD and they have to be straight and the right height and exact spacing, so the eyeball thing does help to a degree. Thanks for all your help.