Most standard build in my eyes is: Boots3->Tear ASAP->Rod of Ages->Glacial+Negatron+Mercs/Sorcs->Frozen Heart+Banshee's Veil->Guardian Angel+Archangel's
This build gives a good amount of damage, HP, Armor and MR. You'll be very tanky, do good damage, have a GA and Banshee's protection, 40% CDR, etc.
However some other items you will want to use, primarily, are Will of the Ancients (always spell vamp never die, good for double AP comps or if your support has enough gold to get one, so you can have a double WotA going), Void Staff (if multiple enemies stack MR).
Main combo with 40% CDR is: QWQEQRQWQEQ Q QWQEQ
During small beats, reposition to chase/kite. During large beats, you might fit in an autoattack. When you've ulted, try to focus a carry but also try to land skills to make the best use of your AoE damage.
Oh, and you can do 9/9/12, 9/0/21 or 0/9/21 for masteries. Offense gives you damage, defense gives you HP and Armor/MR, utlity gives you MS, Gold, Spell Vamp, XP, Health Regen and CDR. Strength of Spirit is a nice reason to go X/X/21, it's a lot of HP regen mid/late for 3 mastery points. I've been using 9/9/12 but previously used 9/0/21 and haven't decided which I prefer.