Probably Ethereum.

It is the one that is perhaps most used. Plenty of smart contracts on it. Almost every ICO conducted has raised funds via Ethereum (or Bitcoin). Most of the transactions for ICO related token sales are on the Ethereum platform. Very economical to move value via Ethererum.

Again, perhaps the only cryptocurrency token/platform that has been tested with real-world examples. Whilst a lot of competing networks are coming out, Ethereum (in my opinion) is still the most used/widely-acceptable crypto currency outside Bitcoin.

With the recent reorganization and focus of Stellar, XLM might also stand a pretty good chance of replacing Ethereum for ICO/Token sales.