Unreleased pokemon usually aren't allowed, but there is nothing wrong with using legendaries. Higher stats don't always mean better, typing and moves are also very important. let's take Raikou for example. It's stats are good an all, and electric typing isn't half bad, but it's not really amazing either. 115 special attack and speed are good, but it's only stat boosting move is calm mind, which is better for tanks then sweepers, and it has to use hidden power for good coverage.
Just saying.
A dragnite parter...Ferrothorn should work, it can take out most pokemon that carry ice attacks. To tell the truth, ice types are actually pretty rare in OU. Most ice moves you'll see in OU are carried by water types and donphan's ice shard. After that, it's just Cloyster and Mamoswine that have ice STAB, both of which are checked/countered by Ferrothorn.
Don't counter the type it's weak to, counter the pokemon that often carry the moves of that type.