After patch 8.3, Sona's early game is rather weak early game, however she scales pretty well after you have obtained a few items.

Here is a "I am in loading screen and need a build in 10 seconds" version of my guild:
build: Frostfang -> Athene -> Tier 2 boots -> archangel (seraph) -> ardent -> rabadon

If you are still reading, I am guessing you have a bit of time to carefully read over the guild. In the following sections, I will explain the different scenario / decisions you might come across during the game. The quick build above isn't always the best choice given the situation you are facing in game.

For this build, we start with Q, then put 1 level on W and E for level 2 and 3 respectively. Afterwards, max in order of R>Q>W>E. Upgrading Q on level 3 and E on level 4 can also be considered if you know your oppoent's jungler is far away from bot and your laning opponents are low.

Flash is a must and there is no alternatives. The importance and significance of flash is obvious and well known.

Ignite gives you quite a bit of damage during your first fights in lane and often it is the difference between a kill or not. In addition, it also reduces the efficiency of healing on the enemy team. Exhaust can be considered an alternative if you plan to sit back during laning phrase or if you think an enemy might be a large threat late game. In general, however, I would say ignite > exhaust for sona.

Summon Aery gives more damage on poke and more heal when you use your abilities, making it a solid choice.

Manaflow band gives you extra mana when you hit your abilities and bonus mana regen when you completed your stacks. Mana regen is always needed and liked on sona.

Transcendence gives cooldown reduction, you reach level 10 roughly the same time you complete athene, combined together, this gives sona a decent first power spike.

Gathering storm helps you get stronger as the game goes on.

Cosmic Insight doesn't look strong on first glance, but the extra 5% on the cap of cooldown reduction will help a lot late game. A 3 second cooldown on Q and W is definitely a nightmare for your opponents.

Biscuit gives extra sustain in lane. This doesn't just apply to health, more so for mana. During Season 8, mana costs of sona's ability has been increased quite a bit while its efficiency has been decreased. This means you would have to use your ability a lot more than often. The mana regen on biscuits is going to be very helpful on keeping you with enough mana to constantly cast your abilities.

Athene's Unholy Grail is a core item

We start with spelltheif + refillable potion. As runes give you biscuits which restores your health and mana, refillable potion would give enough sustain during your laning phrase.

Early Game:
Upgrading spellthief to frostfang will allow you to gain gold much faster, and so it is the first choice upon your first back.

At this point, you should consider your current situation in lane. Buying Tear of the Goddess first is often a good choice as it allows you to use your abilities freely without fearing to run out of mana quickly. However if you think your lane is behind, you can also get fiendish codex before tear as it gives cdr and ap (which boosts your poke and heal).

Also, remember to buy a vision ward if you have extra gold. A ward can make a huge difference when the enemy ganks your lane.

In the usual case, you would buy tears first. On the next few backs, buy Boots of speed and fiendish codex. We then complete fiendish codex into athene's unholy grail. Athene should always be the first item you complete, the unique passive gives you roughly 150 (or even up to 180) extra healing when you cast "w" by the time you complete the item, which pretty much doubles your healing early game. Although it is typically better to have tear early so as to fully stack it for seraph's during mid game. Given the need to scale faster in terms of poke and heal, you can consider getting athene even before buying tears.

Mid Game

Some of you might be confused with the way I aligned my items at the top (sorry I am new to this build website) so I would explain myself here. At this point, you should have frostfang (which should have became eye of frost by now), Tier 1 boots, tear of the goddess and athene's unholy grail. Here are the few choices you can make next and the situations where I recommend you to do so.

[*] If your tear stacks are near completion, consider completing archangel first. 20% cooldown reduction and basically enough mana to ensure you have no more mana issues is really good. Also with the passive (scaling ap from mana) the item should give you roughly 120ap on its own, making it a pretty good item. The seraph shield also helps keep you alive during team fights. Afterwards, get Tier 2 boots and ardent.

Typically I like building boots of swiftness as it allows you to move around really fast if you combine it with your E ability. Sorcerer's Shoes can also be considered if a bit of extra damage is what you would like. Ninja Tabis and Mercury Treads are also good choices if you are facing a team with a lot of AD or AP/CC respectively.

[*] If your tear stacks are still far from 750, I would recommend getting Tier 2 boots next. Shoe choices are the same as above. We then again look at your tear stacks. If you are close to completion now, go for archangel (seraph) first, then ardent. If not, then get components for ardent until your tear stacks are way above 500/750.

Late Game:

At this point, you should have eye of frost, Tier 2 boots, athene, archangel (which typically should be seraphs by now), and ardent.

Personally I like building rabadon's deathcap as the last item. A significant increase in ap will mean a lot more damage and heal with your Q and W abilities respectively.

Alternatives for endgame:
1) Lich Bane as last item:

This will give you quite a bit more damage during team fights at the cost of a not as significant heal/barrier on your W. This item is only a better choice than rabadon if your team really needs that extra damage. I

2) Mikael's Crucible as last item:

This will increase your heal and shields by a bit but the AP this item gives simply makes it not as good of a choice compared to rabadon deathcap/lich bane in most cases. Build this only if one of your team's carry has an intensive need for removing cc. Eg. Your team includes a really fed yi but the enemy is throwing a lot of removable cc on him every fight.

3) getting Rabbadon first, then selling boots and buying lich bane:

This alternative is one that I have used quite a bit in plat elo. By having both rabadon's deathcap and lich bane, your damage and heals during team fight will be insanely strong. The catch is the fact that without boots, you would have to rely heavily on your personal positioning to make sure you can live long enough to make an impact. Seraph can only give a temporary shield during fights.