without a doubt, single tree cold blizz. basically, here's what you do:
1-5: 1 pt in warmth. all other points in ice bolt, dump the beginner staff and get one with +1-3 ice bolt. keep buying mana pots.
6-17: 1 pt static, telekenesis, rest in ice blast. immediately put 2-3 tir runes in a 2-3 socket helm, any helm will do. you should be able to destroy everything with ice blast, static owns normal bosses.
18-23: 1 pt in teleport, rest in glacial spike. if you run normal countess a few times, you should be able to make a stealth armor, tal-eth. use a breastplate from a2.
24-29: all points in blizzard.
30: cold mastery.
31-37: you should be able to put points in both blizzard and cold mastery at the same time due to quest bonuses. if not, leave cm at 1 pt and max blizz first.
50+: with blizz maxxed, raise cm to 7, then glacial spike to max. get an a2 nm merc and use glacial spike/static when he is more than 5 levels behind you and let him get the kills as much as you can.
70: stay in nm baal runs until this level. tp through hell doing the main quests. there are only 5 bosses standing between you and hell meph. your merc should take care of the only possible cold immunes, the council in a3. once you get to meph, refer to the moat trick guide and farm him like a polygamist. you should try to save up 7 ptopazes and put them into a helm and armor. this will be enough to get you started.
refer to the levelling guide in the strategy compendium for where you should be at each level. pay particular attention to lvl 24 and 25.