These are all great, and picking just one can actually be painful. Frostbolt is an excellent early game removaland it complements the already present Fireball and Firelands Portal very well. Fire Plume Phoenix is simply a great 4-drop and usually a good keep in the opening hand with a coin when going second. Blizzard is an excellent board clear and just as excellent as a stall card before the Flamestrike turn. Personally, I would pick the Frostbolt, but each of these cards has a place in the deck.
1. Frostbolt, Fire Plume Phoenix, Blizzard