What's the optimal Suicine hidden power type?

Water due to stab and for the reason you can only use it as an attacker. Still worse than vaporeon.

Bug or Fire would be cool to surprise grass attackers. You can't use them as defenders, but maybe they will change it so you can put legendaries in gyms.

Since Suicune can't be used for defense, you're better off with something that matches its charge move's effectiveness. Water is the obvious one, but Ground or Rock can also be useful, the latter two offering additional coverage options.

Not that Suicune is really that useful in attack unfortunately. I managed to get a perfect IV one with Hidden Power Ground, but I'm struggling to find a good reason to actually power it up.

Rock when birds come back, but unfortunately slightly outclassed by Omastar.

I'm thinking that if someone gets lucky enough to get Hidden Power: Fight and HydroPump, that Suicune might be better than Vaporeon for taking down Tyranitars. Not by a lot, but hitting the double weakness, and then the regular weakness with the charged attack, and resisting steel and fire can help too.

I would be satisfied with Ice, Rock, Fight and Bug in this order.

Ice, because it's double effective against Dragonite and I could imagine Suicune getting an ice charge attack in the future

Rock because it could be useful, if the legendary Birds come back (double effective) + it shares one effectivity (fire) with it's water charge attacks

And Fight because Tyranitar (double weakness + weakness to water)

But... Suicune isn't really an attacker and doesn't fit in any role available now, so I would only power up a 100% one, until it becomes available as a defender (and proves as such)
-> in that case, I would favor Ice or Bug, since they are super effective against grass Types, Zapdos and generalists/popular attackers like Dragonite, Tyranitar and Mewtwo

Water by a wide margin. The STAB effective adds 25% to its overall attack stat, and it matches the charge moves.