Slimes are not necessarily the weakest.

A boy named Marumiya Eishi is killed by a god's mistake and is reborn as a slime. While complaining that it'll end up being a ripoff, he apparently becomes the strongest through an ability given to him by the god, and in another world, he gets along great with a human girl, a vampire girl, an ogre princess, a demon lord, and... isn't he just getting along great with everyone? Well, it's a story where various things happen.