Polkadot is currently the toughest competitor for ethereum compared to other platform blockchains like eos, stellar, waves, neo and it is actually very difficult for polkadot to become ethereum killer considering ethereum still has advantages over this, but if ethereum makes a mistake it's not impossible polkadot will become ethereum killer.
actually, only real comparison to Polkadot is Cosmos network, since they are both offering block-chain interoperability and smart contracts functionality, all others are offering only one, or part of both, but Polkadot and Cosmos seem to be most comprehensive at the moment
would also add Zilliqa, but i do not know why are they that much behind when speaking about smart contract platforms, they already have implemented sharding, similar to what Ethereum is trying with ETH2.0