Aurelion Sol is an intriguing character to play. It's a mage that must consistently position itself close to the enemy team in order to do damage, yet is relatively squishy and often becomes focused because of the damage he can cause when left unchecked. As the newest hero, many of his tactics are still being tested and discovered, meaning his eventual meta will likely change as players learn more about him. What lane is he good in? Jg? Bot? ect. So in light of that I am submitting a guide that reflects my experience with the champion and where I've found consistent success.

This guide is a work in progress so many things may change, but it may give you a starting point for figuring out how you might want to play this champion.

The build I've developed is a pretty big deviation from the AP offensive builds that are suggested. It's based on the logic that, Aurelion tends to be a champion that is often focused as a primary means of dealing with the threat he represents. Tank items have a very pronounced effect on his overall survivability and require champions to invest considerably more skills are time in bringing the celestial dragon down. This is especially true, when dealing with melee assassins, who often expect to win by making a brief trade.

I've found Aurelion to be especially adept at the support role. His passive and W ability make it easy to harass opponents. His Q is a very versatile ability that's good for setting up kills for your ADC, repelling attacks from the other teams ADC, and stopping ganks from the jungle. Use Aurelion Sol as a floating hazard zone, to keep the enemy ADC away from farming, harass often, and stun anyone who steps a millimeter to close to your ADC.

Pros:

Strong Area of Effect Damage
Good Crowd Control
Great Team Fighter
High Mobility

Cons:

Squishy
Often Focused Due to High Threat
Jungle Camps Never Leave You Alone

Celestial Expansion is Aurelion Sol's primary damage ability so I tend to upgrade it first as I use it the most often. It pushes his Center of the Universe stars to their outer limit and increase their damage. Also, your AP scaling increases with each rank, so it really needs to be maxed first to make the most of your items. When combnied with the Crystal Scepter, your stars slow any enemies they contact making it very effective crowd control.

Starsurge is the second ability you should consider maxing. It's your primary form of crowd control and it's ability to continuously expand sets up the potential for game making plays. When combined with Comet of Legend, a well nurtured Starsurge can stun the entire enemy team. Also, like Celestial Expansion, it's AP scaling increases with rank.

Comet of Legend is usually maxed last. While it provides excellent mobility and receives a massive increase in range with each rank, it's usually more important to consider skills that give you offensive capabilities.

Voice of Light is Aurelion Sol's ultimate. It's a line skill shot that does very good damage that scales with 70% of your AP at all ranks. When enemies are within the inner limit of your passive it knocks enemies into your outer limit. This skill is great as an opener, when combined with a Comet of Legend/Starsurge dive, for picking off weak enemies, for getting some breathing room, and should be used whenever a good opportunity presents itself.

Aurelion Sol is one of the most efficient farmers I've played in the game. When positioned properly, he can farm creep waves with alarming speed. Most players tend to position Aurelion in a stationary position where the stars will passively move through the wave. However, I find it more effective to move in a circle around the wave in a clockwise rotation. This causes your stars to impact more creeps more often and increases your clear time significantly. When you encounter massive waves, try this and you'll be surprised how quickly you can farm an entire wave irregardless of it's size.

Core Items:

The single most important offensive/defensive item you have. It allows Aurelion to apply a constant slowing effect to enemies and offers a very good increase in AP and HP. I tend to make this the second item I get.

Because this build focuses on survivability over offense, a Rabadon's Deathcap is a very important item for providing you with the AP you need to remain relevant in the late game phase and tends to be the third item I acquire.

Defensive Items AD:

If you are fighting a team with primarily Attack Damage then these items are some of the most effective at providing good survivability against most AD champions.

If you know the lane you're facing contains a melee assassin you should consider cloth armor as your starting item. The added 15 armor translates in the enemy needing to do 15% more damage to score a kill. In early game this allows you to survive most harassment from AD champs.
Upgrade to a chain vest and buy a second cloth armor as soon as you can, and you'll find most AD players are very manageable in a trade.

Is usually the first item I acquire in these circumstances. It provides a strong boost in HP and Armor, and the constant burn effect provides some offensive output against melee champions that have breached your inner limit.

I usually purchase one of these armor pieces as my third item. All good choices for certain situations. Frozen Heart, is a good choice when dealing with durable AD champions, as the extra mana provides the ability to sustain your damage longer. Thornmail is good for melee assassins or ranged DPS providing you with very good protection and additional offensive capabilities when under attack. Ninja Tabis are good when you need extra mobility but not necessarily a large upgrade in armor. Finally, Randuin's Omen is a good choice if you are facing champions with critical hit heavy builds. Which ever one you choose as your third item, you should pick two more as your fifth and sixth choice.

Whatever you choose, Rabadon's Deathcap should be your foruth priority, as it provides a much needed power spike that will carry you into the later portion of the games. Fill out the rest of your item slots with the above items to further increase your durability as the game progresses.

Defensive Items AP:

Starting out, you should purchase a Null Magic Mantle first to help deal with AP damage in trades.
As soon as you can, upgrade your Null Magic Mantle to a Spectral Cowl and then a Banshees Veil.
The spell shield passive provided by Banshees Veil is an excellent way to mitigate damage, avoid crowd control, and is just an all around excellent choice for a position-based champion like Aurelion.

You should consider either Mercury's Treads or Abyssal Scepter for your third item depending on whether you need more protection from AP damage, or crowd control. The tenacity on Mercury's Treads are very nice for surviving stuns, snares, or slows, while providing more mobility. The Abyssal Scepter provides more protection from AP damage and more AP for your offensive capabilities. The additionally magic penetration is also good for dealing a little more damage to enemies in your inner limit.

The fourth item can be a choice between Abyssal Scepter or Rabadon's Deathcap if you didn't get one earlier, or if you are surviving well and just want your next power spike.

Finally, round out your build with either Spirit Visage, ZZrot's Portal or Abyssal Scepter depending on whether you have could healers or need the most AP protection, a bit of AD/AP protection, or more damage respectively.

Optional:

can substitute for your boot selection if you feel that more mobility may be more valuable than survivability. For example, if you're dealing with a durable champion, it may be wiser to simply have the tools to outrun them, rather than try to out tank them, which you probably wouldn't.