Not Updated For Current Season
Heres my guide on Harry Potter Sona, i've played almost every guide mobafire has to offer for Sona and I believe (and have tested!) this build in normal and ranked games. This build will NOT give you the ingredients to carry a whole team, you're a support. If you want a carry guide go look elsewhere.
Pros / Cons
Very good healer
Ultimate used correctly changes the game
Insane damage end game
Very good harasser
Can't solo anything at low level
Can easily be ganked if flash & ultimate is on cooldown
Now your probably sitting there going WTF 3 Philosopher stones?! But seriously, if the game isn't swinging my way (Haven't got a few kills or assists) I say what the hell and buy another one because the gold and regeneration is insane. The first of your late game items you want is Lichbane, this makes you a very hard auto-hitter, deathcap increases everything about you except your health which I guess is a bonus. Due to the way I play Sona I never find myself being nuked in the first few seconds of a team fight, and if they do, I get a free ultimate on them and get away just as fast as they came in. Zhonya's Hourglass is a perfect way of stopping that annoying carry auto-hitting you down though just in case they are persistent enough to try and get you that way!
During the battle you want to be hitting Q & E as much as you can until the chase, the chase you hit E and make sure you slow them down. Try to use your ultimate with other champions, I've found the most helpful is Miss Fortune, Vladmir, Katarina etc any champ with an AoE ultimate.
Standard masteries for Sona, remember that Greed is good for this build an extra 1g every 10 seconds is a godsend I'm telling you that now you may not think it is but an average game of lets say 50 minutes is 60*50 = 3000 seconds which is a nice sum of 300g per game from one mastery! I think its insane not to get it especially when playing Harry Potter Sona
Fairly standard runes magic pen/CDR and of course mana per 5, your not stacking mana remember so mp5 is your only source of getting mana back! Each spell costs 65 mana so get used to waiting around for mana, espically at the rate Sona eats it. Magic penetration is obviously for your hymn of valor and ultimate (I am up to arguments that there are better runes for Sona so please do leave a comment!) Cooldown reduction is simply to get your CDR down to as small as possible without having to rely on snowball items (Mejai's)
Up for debate on others but I've found so far that flash (best escape mechanism for Sona) and Clairvoyance is the best scouting method known to the game! Not only have I set ganks up with this 55second cooldown spell but I have also avoided a lot of ganks for both me and my fellow team mates
Being the centre of your team is important, don't be at the front or the back, being at the front will obviously get you targeted and at the back your asking for the flankers (xin, eve etc) to come around and 2 shot your face, stay as near as possible to your taunting tank!
As your playing a gold build you don't need many last hits so let your carry that your laning with get them, however, in order to get your first two philosopher stone's you need a cool 1300 odd gold so you need to get them as much as possible till you hit that amount
I would not play this build without serious practise in normal games
In ranked games Sona as your support relies on a lot of things that your team as to do, I use the exact same build but my play becomes a lot less aggressive. I have a really low average of deaths on Sona and a really high average of assists on Sona and that's the way I want it to stay. Please remember though in solo queue that a support needs a good team surrounding it so please don't downvote me when you die 8 times in 5 minutes because your team mate wouldn't help you.