To my knowledge, there are very few ice types in r/s/e, two of which start out as normal, but can be changed to other types (kecleon and castform). While you weren't very specific on which kind of best it is (for battles, for looks, etc.) I'll share my "knowledge" with you (why am I so formal? I gotta stop this.)
Anyways, the best ice type for battles would be walrein, since it has defensive stats and decent attack power. The best for aesthetics would be Glalie (my personal opinion, but I'm sure at least one other person agrees with me. The best hidden ice type is regice, since it's a very cryptic process to get him, and you need to be able to sight read braille (ironic, isn't it?).