I'm still familiarizing mys of with 5e but I've always had a soft spot for Druids and wizards personally. Want heals? Druid. Everything but heals? Wizard. But I'm big on roll play not just mehanics so the mindset of both interest me as well. It boils down to preference, mindset of yourself and the character.

If we're talking spell dueling or raw magic might I'd put my money on the wizard or sorcerer more often than not. Fighting and casting in the Frey of combat? Sorcerer or cleric. (Maybe Druid). Heals? Druid or cleric, bard would be alright. Overal utility? Likely bard.
So it depends on your play style and preference, what gap you want to fill. There really isn't an answer to "best" overall, all have a place and best use.