This guide is essentially my best guess of what the highest win rate build on Support Syndra would be from http://current.lolalytics.com/champion/Syndra/Support/ on January 26, 2017. I updated the runes based on mid lane Syndra's runes in patch 7.24, but the Lolalytics page was gone at that time, so the rest of the build is the same as before.

There are two important things to be aware of. First, High winrate doesn't necessarily mean best. More expensive and later game items tend to have higher win rates simply because losing players don't have the gold to buy them. More common items tend to have lower win rates simply because everyone gets them regardless of if they win or lose. So as always, practice, test, and use your understanding of the game to adjust your builds according to the circumstances. Theoretically, this should be the best build in a game vs generic opponents where you are doing well, but if you are behind or vs specific types of champions, varying your build is important.

Second, there aren't a lot of support Syndra games being played right now, so there are probably a lot of statistical anomalies in the data. One is that at least a couple of bot lane carry Syndra games are grouped with Syndra support. At the moment, Locket of the Iron Solari has a 100% win rate, since the item was purchased in a single game where the player was victorious.

So there is a ton of randomness to the data. But I figure that some data is still better than none, so if you would otherwise have no idea what to build, this guide should be a decent starting point and an idea of the item possibilities.

Eye of the Watchers is by far the most common first item. I didn't list it in the core build because Frost Queen's Claim is higher win rate, but if you are not doing well in lane, Eye is probably the better choice, since it is cheaper.

Since both Rylai's and Liandry's were common items with very high win rates, they are probably some of the best items on Syndra support in any situation.

If the game extends past that point, or if something specific is happening on the enemy team, you should vary your build. Vary it earlier if the situation is more extreme, and later if you can wait past laning phase to deal with it.

Here are some of the best items and some ideas of when to build them:

Abyssal Sceptre - get if the enemy team has a strong magic damage threat or if you're vs double magic damage bot lane (like Ziggs/Zyra for example)

Zhonya's Hourglass - Get if the active will save you from something important or if the enemy team is heavy on AD.

Void Staff - Get if the enemy team, or at least the targets you are focusing, is building a lot of MR. The more MR, the earlier this purchase is justified.

Luden's Echo - Probably the strongest item for pure damage. Get if there's nothing in particular you need to react to on the enemy team.

Morellonomicon - This is a very popular item on support Syndra. It doesn't seem like it's as good as other items, since it's quite expensive, but if the enemy team has a healing threat or if you are running OOM very quckly (and can't merely use your abilities less often) or you really need CDR above all else, get this item. I feel like it's probably outclassed as a support item, and you should be fine with the mana regen from FQC or Eye of the Watchers.

Also, don't forget to upgrade to Sweeping Lens and buy Control Wards!

Summoner Spells:

You should probably take ignite vs healers, but Exhaust seems to be the better all-around spell and is much more common.

Abilities:

I have also seen W max work on Syndra, since leveling up W increases the slow on her W. However, ROX Key recently played Q into E max, and there aren't enough games to have stats on Lolalytics yet.

Unfortunately I am not an experienced Syndra support player, I just like to compile resources and statistics from other places. So you will have to learn the nuances of the gameplay elsewhere. The only other guide I found on support Syndra is at http://www.lolking.net/guides/226310, and has a few more tips and tricks.

Thanks for reading my guide. I hope you liked it. I will do my best to update it as more information becomes available.