If you want a Pokemon that actually has NO WEAKNESSES, you will turn to the only possible option: the Tymano family. They are a family of electric types with the ability levitate, which makes them immune to ground moves. This can be avoided, however, with a Pokemon that has the ability mold breaker. Well, no one usually pays attention to abilities, however, and what are the chances where your opponent will switch into their mold breaker Excadrill to deal with your Eelectross with ground moves, assuming they have one? And Eelectross' stat's aren't even that bad!
On another note, the electric/steel type combo boasts some incredible 11 resistances/immunities, meaning some moves do little to no damage against them. Unfortunatly, it electric/steel has a bunch of weaknesses that leave it in the dust.
On yet ANOTHER note, pure steel types have an incredible 10 weaknesses and one immunity, making them on just the same level as the electric/steel guys.
On the side, Mamoswine, Terrakion, Scrafty, and a lot of other Pokemon all have good STAB moves that are important, I couldn't just name one!