This is THE fastest XP glitch in Fallout 4 after patch 1.5! I averaged about 100,000 XP every 5 minutes with this method.
STEP 1 - Go to any settlement, preferably one that not in use. Build a robot workshop. You're going to need to have a companion with you, this can be literally anyone, just make sure its a companion. Now firstly we are going to build a robot workshop.
STEP 2 - Enter the robot workshop and create a new robot. Go down to torso and you should see it says that the default torso is a Protectron Torso. We need to now create and apply the Assaultron Torso to our robot, once done just exit the workshop.
STEP 3 - Now we need to call over our companion and get them as close to the robot workbench as possible. Talk to your companion and then when it says press Square or X to talk to him, do so. As quickly as you can move away from them and press A or X on your robot workbench before the trade menu shows up.
STEP 4 - Now we need to select the robot we created in STEP 2. BUT you should notice that the trade menu had popped up behind the robot workshop dialog box, this means you're doing the glitch correctly so far. What we need to do is navigate down to the Torso option using ONLY our D-pad. Select where it says Assaultron Torso and you should see the option to apply No Torso Mod. Just keep spamming the A or X button as quick as you can. (It may not look like you're doing anything but you are!) Do this for as long as you want as every time you're pressing the button you're earning XP!
STEP 5 - Back out of the menu and robot workbench completely and you should see your XP bar flooding up like crazy! Repeat before they patch it and enjoy yourselves!