Data was last updated from http://aram.lolalytics.com/champion/Heimerdinger/ during patch 7.23

As always, don't just follow the ideal build without thinking. Adapt your build based on the circumstances. Deathcap is the highest win rate first item, but that's probably not just because raw AP is amazing on Heimer. It's also because Deathcap is the most expensive first item. If you don't have the gold to complete it first, feel free to go with a different item. For example, Rod of Ages is a great item if you need the extra health and mana. Luden's is an excellent heavy AP item to rush that costs less than Deathcap.

One of the chief rules of ARAM is that there are no fixed "best" builds. The team comps vary so much on both sides that there are often great ways to take advantage of the situation by adapting. The items in the "Items by Win Rate" section are ordered by win rate to help you adapt on the fly. Low win rate items aren't necessarily bad items. They are often the items you should build when behind or when facing difficult opponents or team compositions.