We currently run at $10m+ annual sales in the US and are launching in EU shortly. We don't like Amazon due to their complete lack of support and are wondering what other ecommerce marketplaces are available in EU to take their place. (such as Allegro, Bol, Cdiscount, etc...)
As Nikki said, it does depend on the product. The answer also depends on your marketing strategy. Market place as such will not do your marketing you. For end users there is ebay for for pan-European, Tradera for sweden and many other local ones.
In Russia you can check Avito.ru - the biggest marketplace ever. Ozon.ru or Wildberries.ru.
In Ukraine - Rozetka.ua
In Poland - Olx.pl.
If you need the whole list of marketplaces please let me know.
If you can specify the type of your items - my answer could be more specific.