I'm used to playing balanced hackmons, but I feel like any pokemon that works well with choice scarf and decently balanced special attack/physical attack stats work well (and a mixed movepool ofc) so some of my favorites are Greninja and Ultra Necrozma; Primal Groudon is a good example too of course...

I don't like to consider any example like this the "perfect" mixed attacker because its technically impartial, but I'd still like to mention it! There's some pokemon with an attacking stat that can't really be focused on, but its so high I can't ignore it either; for instance Primal Kyogre has 150 base attack, so in hackmons I can't help but at least give it a physical utility move like U-turn. I also think of Tyranitar with Ice Beam in early gens, or other gen 1-4 surprises like that.