I'm surprised this one hasn't been mentioned yet! Empoleon remains the only Pokemon to have the Water/Steel typing, which itself offers 10 resistances, 1 immunity and 3 weaknesses. It's a good combination, but not insane - Electric, Ground and Fighting aren't great weaknesses to have and I recall finding the neutrality to Fire (on my designated Water type) to be an occasional annoyance. It's a combo that I think is quite easy to imagine a new Pokemon having - Water type with a weapon, Water type with armour, Water type who builds... stuff, Steel type who uses water to cool down, rust-themed Pokemon, etc.
I mean even Empoleon doesn't necessarily scream "Steel type" to me? I know the Pokedex mentions sharp blades, it has a royal vibe (and a few royal themed Pokemon like Aegislash and the Galarian Wolves have Steel typings) plus there's the whole trident visage going on, but lots of Pokemon have that but not the type! Honestly given the Pokedex Water/Ice would've been more fitting maybe. Point is, I don't see how it'd be too difficult to create a design for this type combination and I don't think it's too overpowered as a type to warrant holding back (maybe more so now, back in Gen 4 it would've resisted Dark and Ghost which I can see as being maybe a smidge strong on a better Pokemon).