Look at the team comp
Do you have a bunch of AoE on your team? Do you have little to no intiate? Do you have other Aura based champions? Sona works for comps like this, you can initiate for you team, especially if you have an AoE person, the stun locks them down for that person to own them. Her poke/heal makes the bot lane easily sustainable, and the auras help for overall beefiness.
Do you have other High Risk/High Reward toons on your team? Do you have an initiater who will most likely get focused hard? Do you have a hypercarry? Lulu works best for this, w/ her shield, you can help the initiate, and your ADC lane prevent damage. The speed boost she offers is good for your initiate to get to the center of their team fast, not to mention to you have a HP buff/knockback if used properly, which can save any teammate instantly. Her slow is also good, and your have a stealth reveal w/ pixie if they have an akali,twitch,talon,khazix, vayne.