I did a research regarding that, Filecoin is the largest ICO ever with 257 million dollar. Tezos was the second to the list of very successful ICO with the fundraised 232million dollar.
I sure Ton (token developer of Telegram) have largest ever ICO. After launch a second private presale, Bloomberg Technology reports that the company did just that, and has raised a total of $1.7 billion in March between the two sales.
https://www.theverge.com/2018/4/1/17186004/telegram-presale-open-network-app-ico-cryptocurrency-ton