At Level 1: Deep Chill: Reduces the Movement speed of enemies by 35%. Can be effective for chaseing enemies or to slow the enemy chaseing you.
At Level 4: Snowstorm: Because Envenom is going to bee buffed in the next patch, my personal favorite is Snowstorm. Your blizzard is much bigger and can hit multiple targets at the time.
At Level 7: Frostbitten: 15% sounds not very much but trust me it is.
At Level 10: Water Elemtal: Because its hard to hit an enemy with the Ring of Frost, Water Elemental is a better choise.
At Level 13: Storm Front/Ice Block: If a Illidan or Nova is in your enemy team go for Ice Block.Because Methamorphosis is going to be buffed evry illidan player will pick The Hunt and you become a perfect target without Ice Block. Pick Storm Front if Kael'thas is in your or your enemy team. If he is in your team you can do a pefect combe with his Flamestricke and if he's not in your team you can kill him with
this additional range. It's also a good talent to kill fleeing enemies.
At Level 16: Northern Exposure: With this talent you become a perfect assassin causing enemies increasing their damage taken by 25% percent. If you land your Blizzard perfectly your target is dead soon.
At Level 20: Bolt of the Storm: Can be used offensive and deffensive to chase down enemies or get out of a bad (team)fight and save your life.