Neutral damage from most types, nothing from Electricty and resistance to Steel, Rock, and Fire (the latter two common attack tytpes) merits some commendations, I would think. As well, Water and Ground are two good offensive types individually while combined, good for tanking.7
It's the best of the Water/Grounds, I'll give you that. But not the best combo.
Personally, I think Poison/Dark and Ghost/Dark are the best out there.
Poison's only Super-Effective is against Grass, not many of whom make it to the upper-tiers (save for Shaymin Sky Forme, which is uber). Ground is a very common attack type, thanks to Earthquake. Ghost, Poison, and Grass type attacks are also uncommon in any tier, since their range of super-effectiveness are low. Some will have Energy Ball and Grass-Knot as Swampert counters, though those Pokemon probably have some other attacks up their sleeves as well if on higher tiers.18
Poison/Dark - Neutral damage from most types, 2x damage from Ground, no damage from Psychic (but 2x if Miracle Eye is used), and half damage from Grass, Ghost, Dark, and Poison.
And even if Poison does nothing to Steel, it's one of the deadliest types out there.
I rest my case.