I guess it would be a tie between steem and ethereum at three second block speeds compared to bitcoin's typical ten minute blocks. When you might have to wait three to six confirmations for a completed transaction that time adds up.
A bitcoin transaction fee used to be cheap but now the transaction fee is relatively expensive. Ethereum is a little better than bitcoin as transaction cost goes but getting the lowest transaction fee with just enough "gas" to cover the transaction is more art than science.
Steem is probably the best. It's block time is similar to ethereum but the steem blockchain is "freemium" so most transactions such as upvoting and resteeming activity gets recorded on the blockchain without a direct fee. The direct cost is usually bandwidth amount (resource credits after hardfork 20) which most users don't even notice. When there is a "real cost" such as converting steem on blocktrades the cost is still comparably cheaper than ethereum and bitcoin.
Photo Source: steemit.com / @acidyo