Not Updated For Current Season
As always, you can swap the order of the items depending on team comp/enemy comp. You don't need to have SUPER high AP to be an effective Sona, and this build gives you everything you should NEED to ensure you don't fall early or often in your team fights.
I try to go in the order of Archangels, boots, legion, locket, veil, rylai.
Archangels - plenty of mana/mp5/AP to get you started off with a lot of farm/staying power.
Boots - No real need for an explanation here, survivability and movement = win.
Legion - Getting this is essential for stacking your aura's to the max. It's basically giving you half your blue aura, and your green aura simultaneously.
Locket - once you get the locket you no longer have to worry about mana issues, and help a lot more in team fights with the constant regen/heal from twisting auras.
Veil - self explanatory. More HP/MP and the important bubble for added survivability
Rylai - If they have a lot of melee, this is almost essential. The two nearest enemies to you are slowed on activation of your Q. Not to mention the HP/AP boost.