The best replacement for egg nog is boiled custard.

The best alternatives to egg nog are rated as following:
★★★★★  boiled custard
★★★  holly nog
★★  raw egg nog
★★  tom and jerry
★  holiday nog
★  egg nog
★  eggnog

Your alternative for egg nog is not listed? Suggest!
Speaking about egg nog, one probably shouldn't forget about rum.

Other search terms related to egg nog and their ratings are:

★★★★★ rum
southern comfort
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Google Trends

Scale is based on the average worldwide traffic of egg nog in all years. Click on graph for more.
egg nog
boiled custard
holly nog